spectroscopist /spek tros"keuh pist/, n.
/spek tros"keuh pee, spek"treuh skoh'pee/, n.
the science that deals with the use of the spectroscope and with spectrum analysis.
[1865-70; SPECTRO- + -SCOPY]

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Branch of analysis devoted to identifying elements and compounds and elucidating atomic and molecular structure by measuring the radiant energy absorbed or emitted by a substance at characteristic wavelengths of the electromagnetic spectrum (including gamma ray, X ray, ultraviolet, visible light, infrared, microwave, and radio-frequency radiation) on excitation by an external energy source.

The instruments used are spectroscopes (for direct visual observation) or spectrographs (for recording spectra). Experiments involve a light source, a prism or grating to form the spectrum, detectors (visual, photoelectric, radiometric, or photographic) for observing or recording its details, devices for measuring wavelengths and intensities, and interpretation of the measured quantities to identify chemicals or give clues to the structure of atoms and molecules. Helium, cesium, and rubidium were discovered in the mid-19th century by spectroscopy of the Sun's spectrum. Specialized techniques include Raman spectroscopy (see Chandrasekhara Venkata Raman), nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), dynamic reflectance spectroscopy, microwave and gamma ray spectroscopy, and electron spin resonance (ESR). Spectroscopy now also includes the study of particles (e.g., electrons, ions) that have been sorted or otherwise differentiated into a spectrum as a function of some property (such as energy or mass). See also mass spectrometry; spectrometer; spectrophotometry.

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      study of the absorption and emission of light and other radiation by matter, as related to the dependence of these processes on the wavelength of the radiation. More recently, the definition has been expanded to include the study of the interactions between particles such as electrons (electron), protons (proton), and ions (ion), as well as their interaction with other particles as a function of their collision energy. Spectroscopic analysis has been crucial in the development of the most fundamental theories in physics, including quantum mechanics, the special and general theories of relativity, and quantum electrodynamics. Spectroscopy, as applied to high-energy collisions, has been a key tool in developing scientific understanding not only of the electromagnetic force but also of the strong and weak nuclear forces.

      Spectroscopic techniques have been applied in virtually all technical fields of science and technology. Radio-frequency spectroscopy of nuclei in a magnetic field has been employed in a medical technique (medicine) called magnetic resonance (nuclear magnetic resonance) imaging (MRI) to visualize the internal soft tissue of the body with unprecedented resolution. Microwave spectroscopy was used to discover the so-called three-degree blackbody radiation, the remnant of the big bang (big-bang model) (i.e., the primeval explosion) from which the universe is thought to have originated (see below Survey of optical spectroscopy: General principles: Applications (spectroscopy)). The internal structure of the proton and neutron and the state of the early universe up to the first thousandth of a second of its existence is being unraveled with spectroscopic techniques utilizing high-energy particle accelerators. The constituents of distant stars, intergalactic molecules, and even the primordial abundance of the elements before the formation of the first stars can be determined by optical, radio, and X-ray spectroscopy. Optical spectroscopy is used routinely to identify the chemical composition of matter and to determine its physical structure.

      Spectroscopic techniques are extremely sensitive. Single atoms (atom) and even different isotopes (isotope) of the same atom can be detected among 1020 or more atoms of a different species. (Isotopes are all atoms of an element that have unequal mass but the same atomic number. Isotopes of the same element are virtually identical chemically.) Trace amounts of pollutants or contaminants are often detected most effectively by spectroscopic techniques. Certain types of microwave, optical, and gamma-ray spectroscopy are capable of measuring infinitesimal frequency shifts in narrow spectroscopic lines. Frequency shifts as small as one part in 1015 of the frequency being measured can be observed with ultrahigh resolution laser techniques. Because of this sensitivity, the most accurate physical measurements have been frequency measurements.

 Spectroscopy now covers a sizable fraction of the electromagnetic spectrum. The table—> summarizes the electromagnetic spectrum over a frequency range of 16 orders of magnitude. Spectroscopic techniques are not confined to electromagnetic radiation, however. Because the energy E of a photon (a quantum of light) is related to its frequency ν by the relation E = hν, where h is Planck's constant, spectroscopy is actually the measure of the interaction of photons with matter as a function of the photon energy. In instances where the probe particle (subatomic particle) is not a photon, spectroscopy refers to the measurement of how the particle interacts with the test particle or material as a function of the energy of the probe particle.

      An example of particle spectroscopy is a surface analysis technique known as electron energy loss spectroscopy (EELS) that measures the energy lost when low-energy electrons (typically 5–10 electron volts) collide with a surface. Occasionally, the colliding electron loses energy by exciting the surface; by measuring the electron's energy loss, vibrational excitations associated with the surface can be measured. On the other end of the energy spectrum, if an electron collides with another particle at exceedingly high energies, a wealth of subatomic particles is produced. Most of what is known in particle physics (the study of subatomic particles) has been gained by analyzing the total particle production or the production of certain particles as a function of the incident energies of electrons and protons.

      The following sections focus on the methods of electromagnetic spectroscopy, particularly optical spectroscopy. Although most of the other forms of spectroscopy are not covered in detail, they have the same common heritage as optical spectroscopy. Thus, many of the basic principles used in other spectroscopies share many of the general features of optical spectroscopy.

Survey of optical spectroscopy

General principles
Basic features of electromagnetic radiation
      Electromagnetic radiation is composed of oscillating electric (electric field) and magnetic fields (magnetic field) that have the ability to transfer energy through space. The energy propagates as a wave (wave motion), such that the crests and troughs of the wave move in vacuum at the speed of 299,792,458 metres per second. The many forms of electromagnetic radiation appear different to an observer; light is visible to the human eye, while X rays and radio waves are not.

      The distance between successive crests in a wave is called its wavelength. The various forms of electromagnetic radiation differ in wavelength. For example, the visible portion of the electromagnetic spectrum lies between 4 × 10−7 and 8 × 10−7 metre (1.6 × 10−5 and 3.1 × 10−5 inch): red light has a longer wavelength than green light, which in turn has a longer wavelength than blue light. Radio waves can have wavelengths longer than 1,000 metres, while those of high-energy gamma rays can be shorter than 10−16 metre, which is one-millionth of the diameter of an atom. Visible light and X rays are often described in units of angstroms (angstrom) or in nanometres. One angstrom (abbreviated by the symbol Å) is 10−10 metre, which is also the typical diameter of an atom. One nanometre (nm) is 10−9 metre. The micrometre (μm), which equals 10−6 metre, is often used to describe infrared radiation.

      The decomposition of electromagnetic radiation into its component wavelengths is fundamental to spectroscopy. Evolving from the first crude prism spectrographs that separated sunlight into its constituent colours, modern spectrometers have provided ever-increasing wavelength resolution. Large-grating spectrometers (see below Practical considerations: Methods of dispersing spectra (spectroscopy)) are capable of resolving wavelengths as close as 10−3 nanometre, while modern laser techniques can resolve optical wavelengths separated by less than 10−10 nanometre.

      The frequency with which the electromagnetic wave oscillates is also used to characterize the radiation. The product of the frequency (ν) and the wavelength (λ) is equal to the speed of light (c); i.e., νλ = c. The frequency is often expressed as the number of oscillations per second, and the unit of frequency is hertz (Hz), where one hertz is one cycle per second. Since the electromagnetic spectrum spans many orders of magnitude, frequency units are usually accompanied by a Latin prefix to set the scale of the frequency range. (See measurement system: The metric system of measurement: The International System of Units (measurement system) for a table of the prefixes commonly used to denote these scales.)

Basic properties of atoms (atom)
      An isolated atom can be described in terms of certain discrete states called quantum states. Each quantum state has a definite energy associated with it, but several quantum states can have the same energy. These quantum states and their energy levels are calculated from the basic principles of quantum mechanics. For the simplest atom, hydrogen, which consists of a single proton and a single electron, the energy levels have been calculated and tested to an uncertainty of better than one part in 1011, but for atoms with many electrons, the accuracy of the calculations may not be much better than a few percent of the energy of the levels.

      Atomic energy levels are typically measured by observing transitions (transition) between two levels. For example, an atom in its lowest possible energy state (called the ground state) can be excited (excitation) to a higher state only if energy is added by an amount that is equal to the difference between the two levels. Thus, by measuring the energy of the radiation that has been absorbed by the atom, the difference in its energy levels can be determined. The energy levels are identical for atoms of the same type; allowed energies of a particular atom of silver are equal to those for any other atom of the same isotope of silver.

      Other isolated systems, including molecules, ions (charged atoms or molecules), and atomic nuclei, have discrete allowed energies. The analysis of these simple systems is carried out with techniques that are analogous to those that were first applied to simple atomic spectra. More complex structures, such as clusters of atoms, and bulk condensed matter, such as solids and liquids, also have energy levels describable by quantum mechanics. The energy levels in these complex systems, however, are so closely spaced that they smear into a continuous band of energies. Transitions between these bands allow researchers to discern many important properties of a given material. The location and properties of the energy states are often referred to as the electronic structure (electronic configuration) of the material. By comparing spectroscopic measurements to quantum mechanical calculations based on an assumed model of the material, one can use knowledge of a material's electronic structure to determine its physical structure.

      If an atom in its ground state is given some amount of energy so that it is promoted to an excited state, the atom will release that extra energy spontaneously as it moves back into lower states, eventually returning to the ground state. For an isolated atom, the energy is emitted as electromagnetic radiation. The emitted energy E equals the upper-state energy minus the lower-state energy; this energy is usually carried by a single quantum of light (a photon) having a frequency ν in which photon energy (E) is equal to a constant times the frequency, E = hν, where h, Planck's constant, equals 6.626 × 10−34 joule second. This relationship determines the frequencies (and wavelengths, because λ = c/ν) of light emitted by atoms if the energies of the states are known. Conversely, the relationship allows the energy states of an atom to be determined from measurements of its frequency or wavelength spectrum. The analysis of the discrete wavelengths emitted or absorbed by an atom or molecule was historically carried out using prism or grating spectrometers; because of the appearance of the separated light in these instruments, these discrete wavelengths are sometimes called spectral lines.

Historical survey
      The basis for analytical spectroscopy is the discovery, made in 1859 by the German physicist Gustav R. Kirchhoff (Kirchhoff, Gustav Robert), that each pure substance has its own characteristic spectrum. Another German physicist, Joseph von Fraunhofer (Fraunhofer, Joseph von), repeating more carefully an earlier experiment by a British scientist, William Wollaston (Wollaston, William Hyde), had shown in 1814 that the spectrum of the Sun's (Sun) electromagnetic radiation does not grade smoothly from one colour to the next but has many dark lines, indicating that light is missing at certain wavelengths because of absorption. These dark lines, sometimes called Fraunhofer lines, are also collectively referred to as an absorption spectrum. The spectra of materials that were heated in flames or placed in electric-gas discharges were studied by many scientists during the 18th and 19th centuries. These spectra were composed of numerous bright discrete lines, indicating that only certain wavelengths were present in the emitted light. They are called brightline, or emission, spectra.

      Although the possibility that each chemical element has a unique characteristic spectrum had been considered by numerous investigators, the early studies were hampered by the difficulty of obtaining relatively pure substances. Any sample could contain impurities that would result in the simultaneous production of many spectra. By using carefully purified substances, Kirchhoff demonstrated characteristic spectra and initiated the technique of spectroscopic analysis of the chemical composition of matter. The technique was applied by Kirchhoff and his colleague the German chemist Robert Bunsen (Bunsen, Robert Wilhelm) in 1861 to the analysis of the Sun's electromagnetic spectrum and the identification of the chemical elements in the Sun.

 Before the 20th century, there was no theory that could satisfactorily explain the origin of the spectra of the elements or the reason why different elements have different spectra. The quantitative understanding of the elemental spectra needed the development of a fundamentally new physical theory, and the spectra of the simplest atoms played the key role in the development of this theory. Many of the major developments in 20th-century physics were motivated by an ever-increasing accuracy in the measurement of the spectra of the hydrogen atom; highlights include the discovery in 1885 by the Swiss scientist Johann J. Balmer (Balmer, Johann Jakob) that the frequency spectrum of hydrogen followed a simple numerical pattern, later revised by the Swedish physicist Johannes R. Rydberg (Rydberg, Johannes Robert) and given in modern notation as 1/λ = RH (1/22 − 1/n2), where RH is the so-called Rydberg constant for hydrogen (see photograph—>). In 1913 the Danish physicist Niels Bohr (Bohr, Niels) presented the first theoretical model (Bohr atomic model) that could give quantized energy levels that were in quantitative agreement with measurements of the hydrogen spectrum.

      Despite the success of the Bohr theory in describing the hydrogen spectrum, the theory failed badly when applied to the next simplest atom, helium, which contains two electrons. It was also incapable of predicting the likelihood of transitions between energy levels. In 1925–26 a new theory that could explain the discrete, quantum nature of the spectra was developed by the German physicists Werner Heisenberg (Heisenberg, Werner) and Erwin Schrödinger (Schrödinger, Erwin). This theory, known as quantum mechanics, was extended by the Austrian-born Swiss physicist Wolfgang Pauli (Pauli, Wolfgang), the German physicist Max Born (Born, Max), and others. It has been remarkably successful in describing the spectra of complex atoms, ions, simple molecules, and solids.

      As the spectral lines of the hydrogen atom were measured with increased accuracy, greater demands were placed on the theoretical understanding of atomic spectra. The British physicist Paul A.M. Dirac (Dirac, P.A.M.) combined quantum mechanics with the special theory of relativity in 1928 to describe particles moving close to the speed of light. His formulation of relativistic quantum mechanics (relativistic mechanics) provided an explanation for the so-called fine structure of the hydrogen spectrum (see below Foundations of atomic spectra: Hydrogen atom states: Fine and hyperfine structure of spectra (spectroscopy)). At still higher resolution, two energy levels of the hydrogen atom in the first excited state were predicted by Dirac's theory to be exactly the same. In 1947, the American physicists Willis Lamb (Lamb, Willis Eugene, Jr.) and Robert Retherford discovered that the levels actually differ by roughly 109 hertz (see below X-ray and radio-frequency spectroscopy: Radio-frequency spectroscopy: Methods (spectroscopy)). In contrast, the transition frequency between the ground state and the first excited states was calculated as approximately 2.5 × 1015 hertz. Two American physicists, Richard Feynman (Feynman, Richard P.) and Julian Schwinger (Schwinger, Julian Seymour), and a Japanese physicist, Shinichirō Tomonaga (Tomonaga Shin'ichirō), developed yet another refinement to quantum mechanics to explain this measurement. The theory, known as quantum electrodynamics (QED), had its foundations in the discoveries of Dirac, Heisenberg, and Pauli. It is a complete description of the interaction of radiation with matter and has been used to calculate the energy levels of the hydrogen atom to an accuracy of better than 1 part in 1011. No other physical theory has the ability to predict a measurable quantity with such precision, and, as a result of the successes of quantum electrodynamics, the theory has become the paradigm of physical (physics) theories at the microscopic level.

Applications (chemistry)
      Spectroscopy is used as a tool for studying the structures of atoms (atom) and molecules (molecule). The large number of wavelengths emitted by these systems makes it possible to investigate their structures in detail, including the electron configurations of ground and various excited states.

      Spectroscopy also provides a precise analytical method for finding the constituents in material having unknown chemical composition. In a typical spectroscopic analysis, a concentration of a few parts per million of a trace element in a material can be detected through its emission spectrum.

      In astronomy the study of the spectral emission lines of distant galaxies (galaxy) led to the discovery that the universe is expanding (expanding universe) rapidly and isotropically (independent of direction). The finding was based on the observation of a Doppler shift (Doppler effect) of spectral lines. The Doppler shift is an effect that occurs when a source of radiation such as a star moves relative to an observer. The frequency will be shifted in much the same way that an observer on a moving train hears a shift in the frequency of the pitch of a ringing bell at a railroad crossing. The pitch of the bell sounds higher if the train is approaching the crossing and lower if it is moving away. Similarly, light frequencies will be Doppler-shifted up or down depending on whether the light source is approaching or receding from the observer. During the 1920s, the American astronomer Edwin Hubble (Hubble, Edwin Powell) identified the diffuse elliptical and spiral objects that had been observed as galaxies. He went on to discover and measure a roughly linear relationship between the distance of these galaxies from the Earth and their Doppler shift. In any direction one looks, the farther the galaxy appears, the faster it is receding from the Earth.

      Spectroscopic evidence that the universe was expanding was followed by the discovery in 1965 of a low level of isotropic microwave radiation by the American scientists Arno A. Penzias (Penzias, Arno) and Robert W. Wilson (Wilson, Robert Woodrow). The measured spectrum is identical to the radiation distribution expected from a blackbody, a surface that can absorb all the radiation incident on it. This radiation, which is currently at a temperature of 2.73 kelvin (K), is identified as a relic of the big bang (big-bang model) that marks the birth of the universe and the beginning of its rapid expansion.

Practical considerations
General methods of spectroscopy
      Production and analysis of a spectrum usually require the following: (1) a source of light (or other electromagnetic radiation), (2) a disperser to separate the light into its component wavelengths, and (3) a detector to sense the presence of light after dispersion. The apparatus used to accept light, separate it into its component wavelengths, and detect the spectrum is called a spectrometer. Spectra can be obtained either in the form of emission spectra, which show one or more bright lines or bands on a dark background, or absorption spectra, which have a continuously bright background except for one or more dark lines.

      Absorption spectroscopy measures the loss of electromagnetic energy after it illuminates the sample under study. For example, if a light source with a broad band of wavelengths is directed at a vapour of atoms, ions, or molecules, the particles will absorb those wavelengths that can excite them from one quantum state to another. As a result, the absorbed wavelengths will be missing from the original light spectrum after it has passed through the sample. Since most atoms and many molecules have unique and identifiable energy levels, a measurement of the missing absorption lines allows identification of the absorbing species. Absorption within a continuous band of wavelengths is also possible. This is particularly common when there is a high density of absorption lines that have been broadened by strong perturbations by surrounding atoms (e.g., collisions in a high-pressure gas or the effects of near neighbours in a solid or liquid).

      In the laboratory environment, transparent chambers or containers with windows at both ends serve as absorption cells for the production of absorption spectra. Light with a continuous distribution of wavelength is passed through the cell. When a gas or vapour is introduced, the change in the transmitted spectrum gives the absorption spectrum of the gas. Often, absorption cells are enclosed in ovens because many materials of spectroscopic interest vaporize significantly only at high temperatures. In other cases, the sample to be studied need not be contained at all. For example, interstellar molecules can be detected by studying the absorption of the radiation from a background star.

      The transmission properties of the Earth's atmosphere determine which parts of the electromagnetic spectrum of the Sun and other astronomical sources of radiation are able to penetrate the atmosphere. The absorption of ultraviolet and X-ray radiation by the upper atmosphere prevents this harmful portion of the electromagnetic spectrum from irradiating the inhabitants of the Earth. The fact that water vapour, carbon dioxide, and other gases reflect infrared radiation is important in determining how much heat from the Earth is radiated into space. This phenomenon is known as the greenhouse effect since it works in much the same way as the glass panes of a greenhouse; that is to say, energy in the form of visible light is allowed to pass through the glass, while heat in the form of infrared radiation is absorbed and reflected back by it, thus keeping the greenhouse warm. Similarly, the transmission characteristics of the atmosphere are important factors in determining the global temperature of the Earth.

      The second main type of spectroscopy, emission spectroscopy, uses some means to excite the sample of interest. After the atoms or molecules are excited, they will relax to lower energy levels, emitting radiation corresponding to the energy differences, ΔE = hν = hc/λ, between the various energy levels of the quantum system. In its use as an analytical tool, this fluorescence radiation is the complement of the missing wavelengths in absorption spectroscopy. Thus, the emission lines will have a characteristic “fingerprint” that can be associated with a unique atom, ion, or molecule. Early excitation methods included placing the sample in a flame or an electric-arc discharge. The atoms or molecules were excited by collisions with electrons, the broadband light in the excitation source, or collisions with energetic atoms. The analysis of the emission lines is done with the same types of spectrometer as used in absorption spectroscopy.

Types of electromagnetic-radiation sources

Broadband-light sources
      Although flames and discharges provide a convenient method of excitation, the environment can strongly perturb the sample being studied. Excitation based on broadband-light sources in which the generation of the light is separated from the sample to be investigated provides a less perturbing means of excitation. Higher energy excitation corresponds to shorter wavelengths, but unfortunately, there are not many intense sources of ultraviolet and vacuum-ultraviolet radiation, and so excitation in an electron discharge remains a common method for this portion of the spectrum. (The term vacuum ultraviolet refers to the short-wavelength portion of the electromagnetic spectrum where the photons are energetic enough to excite a typical atom from the ground state to ionization. Under these conditions, the light is strongly absorbed by air and most other substances.)

      A typical broadband-light source that can be used for either emission or absorption spectroscopy is a metal filament heated to a high temperature. A typical example is a tungsten light bulb. Because the atoms in the metal are packed closely together, their individual energy levels merge together; the emitted lines then overlap and form a continuous—i.e., nondiscrete—spectrum. Similar phenomena occur in high-pressure arc lamps (arc lamp), in which broadening of spectral lines occurs owing to high collision rates.

      An arc lamp consists of a transparent tube of gases that are excited by an electric discharge. Energetic electrons bombard the atoms, exciting them to either high-energy atomic states or to an ionized state in which the outermost electron is removed from the atom. The radiation that is emitted in this environment is usually a mixture of discrete atomic lines that come from the relaxation of the atoms to lower energy states and continuum radiation resulting from closely spaced lines that have been broadened by collisions with other atoms and the electrons. If the pressure of the gas in the arc lamp is sufficiently high, a large fraction of the light is emitted in the form of continuum radiation.

Line sources
      Light sources that are capable of primarily emitting radiation with discrete, well-defined frequencies are also widely used in spectroscopy. The early sources of spectral emission lines were simply arc lamps or some other form of electrical discharge in a sealed tube of gas in which the pressure is kept low enough so that a significant portion of the radiation is emitted in the form of discrete lines. The Geissler discharge tube, such as the neon lamp commonly used in advertising signs, is an example of such a source. Other examples are hollow cathode lamps and electrodeless lamps driven by microwave radiation. If specific atomic lines are desired, a small amount of the desired element is introduced in the discharge.

laser sources
      Lasers are line sources that emit high-intensity radiation over a very narrow frequency range. The invention of the laser by the American physicists Arthur Schawlow (Schawlow, Arthur L.) and Charles Townes (Townes, Charles Hard) in 1958, the demonstration of the first practical laser by the American physicist Theodore Maiman in 1960, and the subsequent development of laser spectroscopy techniques by a number of researchers revolutionized a field that had previously seen most of its conceptual developments before the 20th century. Intense, tunable (adjustable-wavelength) light sources now span most of the visible, near-infrared, and near-ultraviolet portions of the spectrum. Lasers have been used for selected wavelength bands in the infrared to submillimetre range, and on the opposite end of the spectrum, for wavelengths as short as the soft X-ray region (that of lower energies).

      Typically, light from a tunable laser (examples include dye lasers, semiconductor diode lasers, or free-electron lasers) is directed into the sample to be studied just as the more traditional light sources are used in absorption or emission spectroscopy. For example, in emission (fluorescence) spectroscopy, the amount of light scattered by the sample is measured as the frequency of the laser light is varied. There are advantages to using a laser light source: (1) The light from lasers can be made highly monochromatic (light of essentially one “colour”—i.e., composed of a very narrow range of frequencies). As the light is tuned across the frequency range of interest and the absorption or fluorescence is recorded, extremely narrow spectral features can be measured. Modern tunable lasers can easily resolve spectral features less than 106 hertz wide, while the highest-resolution grating spectrometers have resolutions that are hundreds of times lower. Atomic lines as narrow as 30 hertz out of a transition frequency of 6 × 1014 hertz have been observed with laser spectroscopy. (2) Because the laser light in a given narrow frequency band is much more intense than virtually all broadband sources of light used in spectroscopy, the amount of fluorescent light emitted by the sample can be greatly increased. Laser spectroscopy is sufficiently sensitive to observe fluorescence from a single atom in the presence of 1020 different atoms.

      A potential limitation to the resolution of the spectroscopy of gases is due to the motion of the atoms or molecules relative to the observer. The Doppler shifts (Doppler effect) that result from the motion of the atoms will broaden any sharp spectral features. A cell containing a gas of atoms will have atoms moving both toward and away from the light source, so that the absorbing frequencies of some of the atoms will be shifted up while others will be shifted down. The spectra of an absorption line in the hydrogen atom as measured by normal fluorescence spectroscopy is shown in Figure 1A. The width of the spectral features is due to the Doppler broadening on the atoms (see Figure 1B.

Techniques for obtaining Doppler-free spectra
 The high intensity of lasers allows the measurement of Doppler-free spectra. One method for making such measurements, invented by Theodore Hänsch (Hänsch, Theodor W.) of Germany and Christian Borde of France, is known as saturation spectroscopy (see Figure 2—>). Here, an intense, monochromatic beam of light is directed into the sample gas cell. If the frequency spread of the light is much less than the Doppler-broadened absorption line, only those atoms with a narrow velocity spread will be excited, since the other atoms will be Doppler-shifted out of resonance. Laser light is intense enough that a significant fraction of the atoms resonant with the light will be in the excited state. With this high excitation, the atoms are said to be saturated, and atoms in a saturated state absorb less light.

      If a weaker probe laser beam is directed into the sample along the opposite direction, it will interact with those atoms that have the appropriate Doppler shift to be resonant with the light. In general, these two frequencies will be different so that the probe beam will experience an absorption that is unaffected by the stronger saturating beam. If the laser frequency is tuned to be resonant with both beams (this can happen only when the velocity relative to the direction of the two beams is zero), the intense beam saturates the same atoms that would normally absorb the probe beam. When the frequency of the laser is tuned to the frequency of the atoms moving with zero velocity relative to the laser source, the transmission of the probe beam increases. Thus, the absorption resonance of the atoms, without broadening from the Doppler effect, can be observed. Figure 1C shows the same hydrogen spectra taken with saturation spectroscopy.

      In addition to saturation spectroscopy, there are a number of other techniques that are capable of obtaining Doppler-free spectra. An important example is two-photon spectroscopy, another form of spectroscopy that was made possible by the high intensities available with lasers. All these techniques rely on the relative Doppler shift of counterpropagating beams to identify the correct resonance frequency and have been used to measure spectra with extremely high accuracy. These techniques, however, cannot eliminate another type of Doppler shift.

      This other type of frequency shift is understood as a time dilation effect in the special theory of relativity. A clock moving with respect to an observer appears to run slower than an identical clock at rest with respect to the observer. Since the frequency associated with an atomic transition is a measure of time (an atomic clock), a moving atom will appear to have a slightly lower frequency relative to the frame of reference of the observer. The time dilation can be minimized if the atom's velocity is reduced substantially. In 1985 an American physicist, Steven Chu (Chu, Steven), and his colleagues demonstrated that it is possible to cool free atoms in a vapour to a temperature of 2.5 × 10−4 K, at which the random atomic velocities are about 50,000 times less than at room temperature. At these temperatures the time dilation effect is reduced by a factor of 108, and the Doppler effect broadening is reduced by a factor of 103. Since then, temperatures of 2 × 10-8 K have been achieved with laser cooling.

Pulsed lasers
      Not only have lasers increased the frequency resolution and sensitivity of spectroscopic techniques, they have greatly extended the ability to measure transient phenomena. Pulsed, so-called mode-locked, lasers are capable of generating a continuous train of pulses where each pulse may be as short as 10−14 second. In a typical experiment, a short pulse of light is used to excite or otherwise perturb the system, and another pulse of light, delayed with respect to the first pulse, is used to probe the system's response. The delayed pulse can be generated by simply diverting a portion of the light pulse with a partially reflecting mirror (called a beam splitter). The two separate pulses can then be directed onto the sample under study where the path taken by the first excitation pulse is slightly shorter than the path taken by the second probe pulse. The relative time delay between the two pulses is controlled by slightly varying the path length difference of the two pulses. The distance corresponding to a 10−14-second delay (the speed of light multiplied by the time difference) is three micrometres (1.2 × 10−4 inch).

Methods of dispersing spectra
      A spectrometer, as mentioned above, is an instrument used to analyze the transmitted light in the case of absorption spectroscopy or the emitted light in the case of emission spectroscopy. It consists of a disperser that breaks the light into its component wavelengths and a means of recording the relative intensities of each of the component wavelengths. The main methods for dispersing (dispersion) radiation are discussed here.

  Historically glass prisms (prism) were first used to break up or disperse light into its component colours. The path of a light ray bends (refracts) when it passes from one transparent medium to another—e.g., from air to glass. Different colours (wavelengths) of light are bent through different angles; hence a ray leaves a prism in a direction depending on its colour (see Figure 3—>). The degree to which a ray bends at each interface can be calculated from Snell's law, which states that if n1 and n2 are the refractive indices of the medium outside the prism and of the prism itself, respectively, and the angles i and r are the angles that the ray of a given wavelength makes with a line at right angles to the prism face as shown in Figure 3—>, then the equation n1 sin i = n2 sin r is obtained for all rays. The refractive index of a medium, indicated by the symbol n, is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. Typical values for n range from 1.0003 for air at 0° C and atmospheric pressure, to 1.5–1.6 for typical glasses, to 4 for germanium in the infrared portion of the spectrum.

      Since the index of refraction of optical glasses varies by only a few percent across the visible spectrum, different wavelengths are separated by small angles. Thus, prism instruments are generally used only when low spectral resolution is sufficient.

 At points along a given wavefront (wave front) (crest of the wave), the advancing light wave can be thought of as being generated by a set of spherical radiators, as shown in Figure 4A—>, according to a principle first enunciated by the Dutch scientist Christiaan Huygens (Huygens, Christiaan) and later made quantitative by Fraunhofer. The new wavefront is defined by the line that is tangent to all the wavelets (secondary waves) emitting from the previous wavefront. If the emitting regions are in a plane of infinite extent, the light will propagate along a straight line normal to the plane of the wavefronts. However, if the region of the emitters is bounded or restricted in some other way, the light will spread out by a phenomenon called diffraction.

      Diffraction gratings (diffraction grating) are composed of closely spaced transmitting slits on a flat surface (transmission gratings) or alternate reflecting grooves on a flat or curved surface (reflection gratings).

 If collimated light falls upon a transmission grating, the wavefronts successively pass through and spread out as secondary waves from the transparent parts of the grating. Most of these secondary waves, when they meet along a common path, interfere with each other destructively, so that light does not leave the grating at all angles. At some exit angles, however, secondary waves from adjacent slits of the grating are delayed by exactly one wavelength, and these waves reinforce each other when they meet—i.e., the crests of one fall on top of the other. In this case, constructive interference takes place, and light is emitted in directions where the spacing between the adjacent radiators is delayed by one wavelength (see Figure 4B—>). Constructive interference also occurs for delays of integral numbers of wavelengths. The light diffracts according to the formula mλ = d(sin i − sin r), where i is the incident angle, r is the reflected or transmitted angle, d is the spacing between grating slits, λ is the wavelength of the light, and m is an integer (usually called the order of interference). If light having several constituent wavelengths falls upon a grating at a fixed angle i, different wavelengths are diffracted in slightly different directions and can be observed and recorded separately. Each wavelength is also diffracted into several orders (or groupings); gratings are usually blazed (engraved) so that a particular order will be the most intense. A lens or concave mirror can then be used to produce images of the spectral lines.

      As the grating in a spectrometer is rotated about an axis parallel to the slit axis, the spectral lines are transmitted successively through the instrument. An electronic photodetector placed behind the slit can then be used to measure the amount of light in each part of the spectrum. The advantage of such an arrangement is that photodetectors are extremely sensitive, have a fast time response, and respond linearly to the energy of the light over a wide range of light intensities (see below Optical detectors (spectroscopy)).

      A third class of devices for dispersing spectra are known as interferometers. These instruments divide the light with semitransparent surfaces, producing two or more beams that travel different paths and then recombine. In spectroscopy, the principal interferometers are those developed by the American physicist A.A. Michelson (Michelson, A.A.) (1881) in an attempt to find the luminiferous ether—a hypothetical medium thought at that time to pervade all space—and by two French physicists, Charles Fabry (Fabry, Charles) and Alfred Pérot (1896), specifically for high-resolution spectroscopy.

      In the Michelson interferometer, an incident beam of light strikes a tilted semitransparent mirror and divides the light into a reflected and transmitted wave. These waves continue to their respective mirrors, are reflected, and return to the semitransparent mirror. If the total number of oscillations of the two waves during their separate paths add up to be an integral number just after recombining on the partially reflecting surface of the beam splitter, the light from the two beams will add constructively and be directed toward a detector. This device then acts as a filter that transmits preferentially certain wavelengths and reflects others back to the light source, resulting in a visible interference pattern. A common use of the Michelson interferometer has one mirror mounted upon a carriage so that length of the light path in that branch can be varied. A spectrum is obtained by recording photoelectrically the light intensity of the interference pattern as the carriage is moved when an absorption cell is placed in one of the arms of the interferometer. The resulting signals contain information about many wavelengths simultaneously. A mathematical operation, called a Fourier transform, converts the recorded modulation in the light intensity at the detector into the usual frequency domain of the absorption spectrum (see analysis: Fourier analysis (analysis)). The principal advantage of this method is that the entire spectrum is recorded simultaneously with one detector.

      The Fabry-Pérot interferometer consists of two reflecting mirrors that can be either curved or flat. Only certain wavelengths of light will resonate in the cavity: the light is in resonance with the interferometer if m(λ/2) = L, where L is the distance between the two mirrors, m is an integer, and λ is the wavelength of the light inside the cavity. When this condition is fulfilled, light at these specific wavelengths will build up inside the cavity and be transmitted out the back end for specific wavelengths. By adjusting the spacing between the two mirrors, the instrument can be scanned over the spectral range of interest.

Optical detectors
      The principal detection methods used in optical spectroscopy are photographic (e.g., film), photoemissive (photomultipliers), and photoconductive (semiconductor). Prior to about 1940, most spectra were recorded with photographic (photography, history of) plates or film, in which the film is placed at the image point of a grating or prism spectrometer. An advantage of this technique is that the entire spectrum of interest can be obtained simultaneously, and low-intensity spectra can be easily taken with sensitive film.

      Photoemissive detectors (photoelectric effect) have replaced photographic plates in most applications. When a photon with sufficient energy strikes a surface, it can cause the ejection of an electron from the surface into a vacuum. A photoemissive diode consists of a surface ( photocathode) appropriately treated to permit the ejection of electrons by low-energy photons and a separate electrode (the anode) on which electrons are collected, both sealed within an evacuated glass envelope. A photomultiplier tube has a cathode, a series of electrodes (dynodes), and an anode sealed within a common evacuated envelope. Appropriate voltages applied to the cathode, dynodes, and anode cause electrons ejected from the cathode to collide with the dynodes in succession. Each electron collision produces several more electrons; after a dozen or more dynodes, a single electron ejected by one photon can be converted into a fast pulse (with a duration of less than 10−8 second) of as many as 107 electrons at the anode. In this way, individual photons can be counted with good time resolution.

      Other photodetectors include imaging tubes (e.g., television cameras), which can measure a spatial variation of the light across the surface of the photocathode, and microchannel plates, which combine the spatial resolution of an imaging tube with the light sensitivity of a photomultiplier. A night vision device consists of a microchannel plate multiplier in which the electrons at the output are directed onto a phosphor screen and can then be read out with an imaging tube.

      Solid-state detectors (solid-state detector) such as semiconductor photodiodes detect light by causing photons to excite electrons from immobile, bound states of the semiconductor (the valence band) to a state where the electrons are mobile (the conduction band). The mobile electrons in the conduction band and the vacancies, or “holes,” in the valence band can be moved through the solid with externally applied electric fields, collected onto a metal electrode, and sensed as a photoinduced current. Microfabrication techniques developed for the integrated-circuit semiconductor industry are used to construct large arrays of individual photodiodes closely spaced together. The device, called a charge-coupled device (CCD), permits the charges that are collected by the individual diodes to be read out separately and displayed as an image.

Foundations of atomic spectra

Basic atomic structure
      The emission and absorption spectra of the elements depend on the electronic (electron) structure of the atom. An atom consists of a number of negatively charged electrons bound to a nucleus containing an equal number of positively charged protons (proton). The nucleus (nucleon) contains a certain number (Z) of protons and a generally different number (N) of neutrons (neutrino). The diameter of a nucleus depends on the number of protons and neutrons and is typically 10−14 to 10−15 metre (3.9 × 10−13 to 3.9 × 10−14 inch). The distribution of electrons around the nuclear core is described by quantum mechanics.

      The chemical and spectroscopic properties of atoms and ions are primarily determined by their electronic structure (electronic configuration)—i.e., by the number and arrangement of electrons surrounding their nucleus. Typical energies of electrons within an atom range from a few electron volts to a few thousand electron volts. Chemical reactions and other processes occurring in spectroscopic sources usually involve energy exchanges on this order of magnitude. Processes that occur within nuclei (e.g., electromagnetic transitions between energy states of the nucleus, beta decay, alpha decay, and electron capture) typically involve energies ranging from thousands to millions of electron volts; hence the internal state of nuclei are nearly unaffected by the usual processes occurring in chemical reactions, light absorption, and light sources. On the other hand, nuclear magnetic moments can be oriented by light through their coupling to the atom's electrons. A process known as optical pumping, in which the atom is excited with circularly polarized light, is used to orient the spin of the nucleus.

      The forces (Coulomb force) holding an atom together are primarily the electrostatic attractive forces between the positive charges in the nucleus and the negative charge of each electron. Because like charges repel one another, there is a significant amount of electrical repulsion of each electron by the others. Calculation of the properties of the atom first require the determination of the total internal energy of the atom consisting of the kinetic energy of the electrons and the electrostatic and magnetic (magnetism) energies between the electrons and between the electrons and the nucleus.

      The size scale of the atom is determined by the combination of the fact that the atom prefers to be in a state of minimum energy and the Heisenberg uncertainty principle. The Heisenberg uncertainty principle states that the uncertainty in the simultaneous determination of the position and the momentum (mass times velocity) of a particle along any direction must be greater than Planck's constant. If an electron is bound close to the nucleus, the electrostatic energy decreases inversely with the average distance between the electron and the proton. Lower electrostatic energy corresponds to a more compact atom and, hence, smaller uncertainty in the position of the electron. On the other hand, if the electron is to have low kinetic energy, its momentum and its uncertainty in momentum must be small. According to the Heisenberg principle, if the uncertainty in momentum is small, its uncertainty in position must be large, thus increasing the electrostatic energy. The actual structure of the atom provides a compromise of moderate kinetic and electrostatic energies in which the average distance between the electron and the nucleus is the distance that minimizes the total energy of the atom.

      Going beyond this qualitative argument, the quantitative properties of atoms are calculated by solving the Schrödinger (Schrödinger equation) wave (wave function) equation, which provides the quantum mechanical description of an atom. The solution of this equation for a specified number of electrons and protons is called a wavefunction and yields a set of corresponding eigenstates (energy state). These eigenstates are analogous to the frequency modes of a vibrating violin string (e.g., the fundamental note and the overtones), and they form the set of allowed energy states of the atom. These states of the electronic structure of an atom will be described here in terms of the simplest atom, the hydrogen atom.

Hydrogen atom states
      The hydrogen atom is composed of a single proton and a single electron. The solutions to the Schrödinger equation are catalogued in terms of certain quantum numbers (quantum number) of the particular electron state. The principal quantum number is an integer n that corresponds to the gross energy states (energy state) of the atom. For the hydrogen atom, the energy state En is equal to −(me4)/(2ℏ2n2) = −hcR/n2, where m is the mass of the electron, e is the charge of the electron, c is the speed of light, h is Planck's constant, ℏ = h/2π, and R is the Rydberg constant. The energy scale of the atom, hcR, is equal to 13.6 electron volts. The energy is negative, indicating that the electron is bound to the nucleus where zero energy is equal to the infinite separation of the electron and proton. When an atom makes a transition between an eigenstate of energy Em to an eigenstate of lower energy En, where m and n are two integers, the transition is accompanied by the emission of a quantum of light whose frequency is given by ν =|EmEn|/h = hcR(1/n2 − 1/m2). Alternatively, the atom can absorb a photon of the same frequency ν and be promoted from the quantum state of energy En to a higher energy state with energy Em. The Balmer series, discovered in 1885, was the first series of lines whose mathematical pattern was found empirically. The series corresponds to the set of spectral lines where the transitions are from excited states with m = 3,4,5, . . . to the specific state with n = 2. In 1890 Rydberg (Rydberg, Johannes Robert) found that the alkali atoms had a hydrogen-like spectrum that could be fitted by series formulas that are a slight modification of Balmer's formula: E = hν = hcR[1/(na)2 − 1/(mb)2], where a and b are nearly constant numbers called quantum defects.

Angular momentum quantum numbers
      There are a set of angular momentum quantum numbers associated with the energy states of the atom. In terms of classical physics, angular momentum is a property of a body that is in orbit or is rotating about its own axis. It depends on the angular velocity and distribution of mass around the axis of revolution or rotation and is a vector quantity with the direction of the angular momentum along the rotation axis. In contrast to classical physics, where an electron's orbit can assume a continuous set of values, the quantum mechanical angular momentum is quantized. Furthermore, it cannot be specified exactly along all three axes simultaneously. Usually, the angular momentum is specified along an axis known as the quantization axis, and the magnitude of the angular momentum is limited to the quantum values √(l(l + 1)) (ℏ), in which l is an integer. The number l, called the orbital quantum number, must be less than the principal quantum number n, which corresponds to a “shell” of electrons. Thus, l divides each shell (shell atomic model) into n subshells consisting of all electrons of the same principal and orbital quantum numbers.

      There is a magnetic quantum number also associated with the angular momentum of the quantum state. For a given orbital momentum quantum number l, there are 2l + 1 integral magnetic quantum numbers ml ranging from −l to l, which restrict the fraction of the total angular momentum along the quantization axis so that they are limited to the values mlℏ. This phenomenon is known as space quantization and was first demonstrated by two German physicists, Otto Stern (Stern, Otto) and Walther Gerlach (Gerlach, Walther).

      Elementary particles such as the electron and the proton also have a constant, intrinsic angular momentum in addition to the orbital angular momentum. The electron behaves like a spinning top, with its own intrinsic angular momentum of magnitude s = √((1/2)(1/2 + 1)) (ℏ), with permissible values along the quantization axis of msh = ±(1/2)ℏ. There is no classical-physics analogue for this so-called spin-angular (spin) momentum: the intrinsic angular momentum of an electron does not require a finite (nonzero) radius, whereas classical physics demands that a particle with a nonzero angular momentum must have a nonzero radius. Electron-collision studies with high-energy accelerators show that the electron acts like a point particle down to a size of 10−15 centimetre, one hundredth of the radius of a proton.

      The four quantum numbers n, l, ml, and ms specify the state of a single electron in an atom completely and uniquely; each set of numbers designates a specific wavefunction (i.e., quantum state) of the hydrogen atom. quantum mechanics specifies how the total angular momentum is constructed from the component angular momenta. The component angular momenta add as vectors to give the total angular momentum of the atom. Another quantum number, j, representing a combination of the orbital angular momentum quantum number l, and the spin angular momentum quantum number s can have only discrete values within an atom: j can take on positive values only between l + s and |ls| in integer steps. Because s is 1/2 for the single electron, j is 1/2 for l = 0 states, j = 1/2 or 3/2 for l = 1 states, j = 3/2 or 5/2 for l = 2 states, and so on. The magnitude of the total angular momentum of the atom can be expressed in the same form as for the orbital and spin momenta: √(j( j + 1)) (ℏ) gives the magnitude of the total angular momentum; the component of angular momentum along the quantization axis is mjℏ, where mj can have any value between +j and −j in integer steps. An alternative description of the quantum state can be given in terms of the quantum numbers n, l, j, and mj.

  The electron distribution of the atom is described as the square of the absolute value of the wavefunction. The probability of finding an electron at a given point in space for several of the lower energy states of the hydrogen atom is shown in Figure 5—>. It is important to note that the electron density plots should not be thought of as the time-averaged locations of a well-localized (point) particle orbiting about the nucleus. Rather, quantum mechanics describes the electron with a continuous wavefunction in which the location of the electron should be considered as spread out in space in a quantum “fuzz ball” as depicted in Figure 5—>.

Fine and hyperfine structure of spectra
      Although the gross energies of the electron in hydrogen are fixed by the mutual electrostatic attraction of the electron and the nucleus, there are significant magnetic effects (magnetism) on the energies. An electron has an intrinsic magnetic dipole moment and behaves like a tiny bar magnet aligned along its spin axis. Also, because of its orbital motion within the atom, the electron creates a magnetic field in its vicinity. The interaction of the electron's magnetic moment with the magnetic field created by its motion (the spin-orbit interaction) modifies its energy and is proportional to the combination of the orbital angular momentum and the spin angular momentum. Small differences in energies of levels arising from the spin-orbit interaction sometimes cause complexities in spectral lines that are known as the fine structure. Typically, the fine structure is on the order of one-millionth of the energy difference between the energy levels given by the principal quantum numbers.

      The hyperfine structure is the result of two effects: (1) the magnetic interactions between the total (orbital plus spin) magnetic moment of the electron and the magnetic moment of the nucleus and (2) the electrostatic interaction between the electric quadrupole moment of the nucleus and the electron (see also below X-ray and radio-frequency spectroscopy: Radio-frequency spectroscopy: Origins (spectroscopy)).

The periodic table
Quantum behaviour of fermions and bosons
      In any atom, no two electrons have the same set of quantum numbers. This is an example of the Pauli exclusion principle; for a class of particles called fermions (fermion) (named after Enrico Fermi, the Italian physicist), it is impossible for two identical fermions to occupy the same quantum state. Fermions have intrinsic spin values of 1/2, 3/2, 5/2, and so on; examples include electrons, protons, and neutrons.

      There is another class of particles called bosons (boson), named after the Indian physicist S.N. Bose (Bose, Satyendra Nath), who with Einstein worked out the quantum statistical properties for these particles. Bosons all have integral intrinsic angular momentum—i.e., s = 0, 1, 2, 3, 4, and so on. Unlike fermions, bosons not only can but prefer to occupy identical quantum states. Examples of bosons include photons that mediate the electromagnetic force, the Z (Z particle) and W (W particle) particles that mediate the weak nuclear force, and gluons (gluon) that mediate the strong nuclear force (see subatomic particle).

      This astounding relationship between a particle's spin and its quantum behaviour can be proved mathematically using the assumptions of quantum field theory. Composite particles such as helium-4 (4He) atoms (an isotope of helium with two protons and two neutrons) act as bosons, whereas helium-3 (3He) atoms (two protons and one neutron) act as fermions at low energies. Chemically, the atoms behave nearly identically, but at very low temperatures their properties are remarkably different.

Electron configurations
 Since electrons are fermions, they must occupy different quantum states of the atom. This profoundly affects the way complex atoms are structured. The periodic table of the elements, first developed independently by Dmitri Ivanovich Mendeleyev of Russia and Lothar Meyer of Germany, can be explained crudely by the sequential filling of hydrogen-like eigenstates. This table lists the elements (chemical element) in rows in order of increasing atomic number; the elements in the same column have similar chemical properties (see Figure 6—>). For an understanding of how elements fit into the periodic table, consider the hydrogen atom, consisting of a singly charged atomic nucleus and one electron. The hydrogen atom in its ground state occupies the n = 1, l = 0, ml = 0, and either the ms = + 1/2 or - 1/2 state; these numbers specify the resulting configuration (electronic configuration), or arrangement, of electrons of a hydrogen atom in its ground state. If a positive charge is added to the nucleus along with a second external electron, the second electron will occupy the lowest energy state, again n = 1, l = 0, ml = 0, but with ms opposite from that of the first electron (otherwise both electrons would have the same set of quantum numbers, and this would violate the Pauli exclusion principle). The resulting configuration is that of helium in its ground state. If both states are occupied by electrons, the n = 1 shell is filled or closed. This closed shell is relatively stable and difficult to excite or ionize; helium is the first of the inert, or noble, gases. If a third electron and proton pair is added to make a lithium atom, the electron cannot occupy the n = 1 shell. The lowest allowed energy state for the third electron is the n = 2 state. For this value of n, the orbital quantum number l can be either 0 or 1, but the state for l = 0 has slightly lower energy. The quantum numbers of the third electron are then n = 2, l = 0, ml = 0, ms = ±1/2. The inner n = 1 shell is relatively stable and remains inert in chemical processes while the chemical and spectroscopic behaviour of this atom is similar in many ways to that of hydrogen, since lithium has one outer electron around a closed, tightly bound shell.

 Addition of the next electron and proton to produce a beryllium atom completes the subshell with n = 2, l = 0. The beryllium atom is analogous to helium in that both atoms have two outer electrons, but the atom is not chemically similar to helium. The reason is that the n = 2 shell is not filled because an electron with n = 2 can also have l = 1. Outside the inner shell n = 1, there are six possible electron states with l = 1 because an electron can have any combination of ml = 1, 0, or −1, and ms = +1/2 or −1/2. As successive electrons are added to yield boron, carbon, nitrogen, oxygen, fluorine, and neon, the electrons take quantum numbers n = 2, l = 1, and all possible different combinations of ml and ms, until a total of six have been added. This completes the n = 2 shell, containing a total of eight electrons in its two subshells. The resulting atom neon, the second of the noble gases, is also chemically stable and similar to helium since the electrons' shells are complete. Increasingly complex atoms are built up in the same manner; chemical similarities exist when the same number of electrons occupy the last partially or completely filled shell (shell atomic model), as shown in the table—.

 As a shorthand method of indicating the electron configurations (orbital) of atoms and ions, the letters s, p, d, f, g, h, . . . are used to denote electrons having, respectively, l = 0, 1, 2, 3, 4, 5, . . . . A number prefixed to the letters gives the value for n, and a superscript to the right of each letter indicates the number of electrons with those values of n and l. For example, the configuration 2s1 represents a single electron with n = 2, l = 0. The configuration 1s22s22p3 represents two electrons with n = 1, l = 0, two electrons with n = 2, l = 0, and three electrons with n = 2, l = 1. The ground state configurations of the first portion of the periodic table is given in the table—>.

Total orbital angular momentum and total spin angular momentum
      For atoms in the first three rows and those in the first two columns of the periodic table, the atom can be described in terms of quantum numbers giving the total orbital angular momentum and total spin angular momentum of a given state. The total orbital angular momentum is the sum of the orbital angular momenta from each of the electrons; it has magnitude √(L(L + 1)) (ℏ), in which L is an integer. The possible values of L depend on the individual l values and the orientations of their orbits for all the electrons composing the atom. The total spin momentum has magnitude √(S(S + 1)) (ℏ), in which S is an integer or half an odd integer, depending on whether the number of electrons is even or odd. The possible value of the total spin angular momentum can be found from all the possible orientations of electrons within the atom. In summing the L and S values, only the electrons in unfilled shells (typically the outermost, or valence, shell) need be considered: in a closed subshell, there are as many electrons with spins oriented in one direction as there are with spins in the opposite direction, with the result that their orbital and spin momenta add up to zero. Thus, only electrons in unfilled shells contribute angular momentum to the whole atom. For light atoms and heavier atoms with just a few electrons outside the inner closed shells, the total angular momentum is approximately given by the vector sum of the total of orbital angular momentum and the total spin angular momentum. The total angular momentum has the magnitude √(J(J + 1)) (ℏ), in which J can take any positive value from L + S to |LS| in integer steps; i.e., if L = 1 and S = 3/2, J can be 5/2, 3/2, or 1/2. The remaining quantum number, mJ, specifies the orientation of the atom as a whole; mJ can take any value from +J to −J in integer steps. A term is the set of all states with a given configuration: L, S, and J.

      If the total angular momentum can be expressed approximately as the vector sum of the total orbital and spin angular momenta, the assignment is called the L-S coupling, or Russell-Saunders coupling (after the astronomer Henry Norris Russell and the physicist Frederick A. Saunders, both of the United States).

      For heavier atoms, magnetic interactions among the electrons often contrive to make L and S poorly defined. The total angular momentum quantum numbers J and mJ remain constant quantities for a given state of an atom, but their values can no longer be generated by the addition of the L and S values. A coupling scheme known as jj coupling is sometimes applicable. In this scheme, each electron n is assigned an angular momentum j composed of its orbital angular momentum l and its spin s. The total angular momentum J is then the vector addition of j1 + j2 + j3 + . . . , where each jn is due to a single electron.

Atomic transitions
      An isolated atom or ion in some excited state spontaneously relaxes to a lower state with the emission of one or more photons (photon), thus ultimately returning to its ground state. In an atomic spectrum, each transition corresponding to absorption or emission of energy will account for the presence of a spectral line. Quantum mechanics prescribes a means of calculating the probability of making these transitions. The lifetimes of the excited states depend on specific transitions of the particular atom, and the calculation of the spontaneous transition between two states of an atom requires that the wavefunctions of both states be known.

      The possible radiative transitions are classified as either allowed or forbidden, depending on the probability of their occurrence. In some instances, as, for example, when both the initial and final states have a total angular momentum equal to zero, there can be no single photon transition between states of any kind. The allowed transitions obey certain restrictions, known as selection rules (selection rule): the J value of the atom can change by unity or zero, and if L and S are well defined within the atom, the change in L is also restricted to 0 or ±1 while S cannot change at all. The time required for an allowed transition varies as the cube of the wavelength of the photon; for a transition in which a photon of visible light (wavelength of approximately 500 nanometres) is emitted, a characteristic emission time is 1–10 nanoseconds (10−9 second).

      Forbidden transitions proceed slowly compared to the allowed transitions, and the resulting spectral emission lines are relatively weak. For atoms in about the first third of the periodic table, the L and S selection rules provide useful criteria for the classification of unknown spectral lines. In heavier atoms, greater magnetic interactions among electrons cause L and S to be poorly defined, and these selection rules are less applicable. Occasionally, excited states are found that have lifetimes much longer than the average because all the possible transitions to lower energy states are forbidden transitions. Such states are called metastable (metastable state) and can have lifetimes in excess of minutes.

Perturbations of levels
      The energies of atomic levels are affected by external magnetic and electric fields in which atoms may be situated. A magnetic field causes an atomic level to split into its states of different mJ, each with slightly different energy; this effect is known as the Zeeman effect (after Pieter Zeeman, a Dutch physicist). The result is that each spectral line separates into several closely spaced lines. The number and spacing of such lines depend on the J values for the levels involved; hence the Zeeman effect is often used to identify the J values of levels in complex spectra. The corresponding effect of line splitting caused by the application of a strong electric field is known as the Stark effect.

      Small modifications to electronic energy levels arise because of the finite mass, nonzero volume of the atomic nucleus and the distribution of charges and currents within the nucleus. The resulting small energy changes, called hyperfine structure, are used to obtain information about the properties of nuclei and the distribution of the electron clouds near nuclei. Systematic changes in level positions are seen as the number of neutrons in a nucleus is increased. These effects are known as isotope shifts and form the basis for laser isotope separation. For light atoms, the isotope shift is primarily due to differences in the finite mass of the nucleus. For heavier atoms, the main contribution comes from the fact that the volume of the nucleus increases as the number of neutrons increases. The nucleus may behave as a small magnet because of internal circulating currents; the magnetic fields produced in this way may affect the levels slightly. If the electric field outside the nucleus differs from that which would exist if the nucleus were concentrated at a point, this difference also can affect the energy levels of the surrounding electrons (see below X-ray and radio-frequency spectroscopy: Radio-frequency spectroscopy (spectroscopy)).

Steven Chu

Molecular spectroscopy

General principles
      A molecule is a collection of positively charged atomic nuclei surrounded by a cloud of negatively charged electrons (electron). Its stability results from a balance among the attractive and repulsive forces of the nuclei and electrons. A molecule is characterized by the total energy resulting from these interacting forces. As is the case with atoms, the allowed energy states of a molecule are quantized (see above Basic properties of atoms (spectroscopy)).

      Molecular spectra result from either the absorption or the emission of electromagnetic radiation as molecules undergo changes from one quantized energy state to another. The mechanisms involved are similar to those observed for atoms but are more complicated. The additional complexities are due to interactions of the various nuclei with each other and with the electrons, phenomena which do not exist in single atoms. In order to analyze molecular spectra it is necessary to consider simultaneously the effects of all the contributions from the different types of molecular motions and energies. However, to develop a basic understanding it is best to first consider the various factors separately.

      There are two primary sets of interactions that contribute to observed molecular spectra. The first involves the internal motions of the nuclear framework of the molecule and the attractive and repulsive forces among the nuclei and electrons. The other encompasses the interactions of nuclear magnetic and electrostatic moments with the electrons and with each other.

      The first set of interactions can be divided into the three categories given here in decreasing order of magnitude: electronic, vibrational, and rotational. The electrons in a molecule possess kinetic energy due to their motions and potential energy arising from their attraction by the positive nuclei and their mutual repulsion. These two energy factors, along with the potential energy due to the mutual electrostatic repulsion of the positive nuclei, constitute the electronic energy of a molecule. Molecules are not rigid structures, and the motion of the nuclei within the molecular framework gives rise to vibrational energy levels. In the gas phase, where they are widely separated relative to their size, molecules can undergo free rotation and as a result possess quantized amounts of rotational energy. In theory, the translational energy of molecules through space is also quantized, but in practice the quantum effects are so small that they are not observable, and the motion appears continuous. The interaction of electromagnetic radiation with these molecular energy levels constitutes the basis for electron spectroscopy, visible, infrared (IR) and ultraviolet (UV) spectroscopies, Raman spectroscopy, and gas-phase microwave spectroscopy.

      The second set of molecular interactions form the basis for nuclear magnetic resonance (NMR) spectroscopy, electron spin resonance (ESR) spectroscopy, and nuclear quadrupole resonance (NQR) spectroscopy. The first two arise, respectively, from the interaction of the magnetic moment of a nucleus or an electron with an external magnetic field. The nature of this interaction is highly dependent on the molecular environment in which the nucleus or electron is located. The latter is due to the interaction of a nuclear electric quadrupole moment with the electric field generated by the surrounding electrons; they will not be discussed in this article.

      Molecular spectra are observed when a molecule undergoes the absorption or emission of electromagnetic radiation with a resulting increase or decrease in energy. There are limitations, imposed by the laws of quantum mechanics, as to which pairs of energy levels can participate in energy changes and as to the extent of the radiation absorbed or emitted. The first condition for the absorption of electromagnetic radiation by a molecule undergoing a transition from a lower energy state, Elo, to a higher energy state, Ehi, is that the frequency of the absorbed radiation must be related to the change in energy by EhiElo = hν, where ν is radiation frequency and h is Planck's constant. Conversely, the application of electromagnetic radiation of frequency ν to a molecule in energy state Ehi can result in the emission of additional radiation of frequency ν as the molecule undergoes a transition to state Elo. These two phenomena are referred to as induced absorption and induced emission (stimulated emission), respectively. Also a molecule in an excited (high) energy state can spontaneously emit electromagnetic radiation, returning to some lower energy level without the presence of inducing radiation.

Theory of molecular spectra
      Unlike atoms in which the quantization of energy results only from the interaction of the electrons with the nucleus and with other electrons, the quantization of molecular energy levels and the resulting absorption or emission of radiation involving these energy levels encompasses several mechanisms. In theory there is no clear separation of the different mechanisms, but in practice their differences in magnitude allow their characterization to be examined independently. Using the diatomic molecule as a model, each category of energy will be examined.

Rotational energy states
      In the gas phase, molecules are relatively far apart compared to their size and are free to undergo rotation around their axes. If a diatomic molecule is assumed to be rigid (i.e., internal vibrations are not considered) and composed of two atoms of masses m1 and m2 separated by a distance r, it can be characterized by a moment of inertia (inertia, moment of) I = μr2, where μ, the reduced mass, is given as μ = m1m2/(m1 + m2). Application of the laws of quantum mechanics to the rotational motion of the diatomic molecule shows that the rotational energy is quantized and is given by EJ = J(J + 1)(h2/8π2I), where h is Planck's constant and J = 0, 1, 2, . . . is the rotational quantum number. Molecular rotational spectra originate when a molecule undergoes a transition from one rotational level to another, subject to quantum mechanical selection rules (selection rule). Selection rules are stated in terms of the allowed changes in the quantum numbers that characterize the energy states. For a transition to occur between two rotational energy levels of a diatomic molecule, it must possess a permanent dipole moment (this requires that the two atoms be different), the frequency of the radiation incident on the molecule must satisfy the quantum condition EJEJ = hν, and the selection rule ΔJ = ±1 must be obeyed. For a transition from the energy level denoted by J to that denoted by J + 1, the energy change is given by hν = EJ + 1EJ = 2(J + 1)(h2/8π2I) or ν = 2B(J + 1), where B = h/8π2I is the rotational constant of the molecule.

Vibrational energy states
      The rotational motion of a diatomic molecule can adequately be discussed by use of a rigid-rotor model. Real molecules are not rigid; however, the two nuclei are in a constant vibrational motion relative to one another. For such a nonrigid system, if the vibrational motion is approximated as being harmonic in nature, the vibrational energy, Ev, equals (v + 1/2)hν0, where v = 0, 1, 2, . . . is the vibrational quantum number, ν0 = (1/2π)(k/μ)1/2, and k is the force constant of the bond, characteristic of the particular molecule. The necessary conditions for the observation of a vibrational spectrum for a diatomic molecule are the occurrence of a change in the dipole moment of the molecule as it undergoes vibration (homonuclear diatomic molecules are thus inactive), conformance to the selection rule Δv = ±1, and the frequency of the radiation being given by ν = (Ev + 1Ev)/h.

Electronic energy states
 Unlike the atom where the system is centrosymmetric (see above Foundations of atomic spectra: Basic atomic structure (spectroscopy)), the energy relationships among the nuclei and electrons in a diatomic molecule are more complex and are difficult to characterize in an exact manner. One commonly used method for consideration of the electronic energy states of a diatomic molecule is the molecular orbital (MO) approach. In this description the electronic wavefunctions of the individual atoms constituting the molecule, called the atomic orbitals (AOs), are combined, subject to appropriate quantum mechanical and symmetry considerations, to form a set of molecular orbitals whose domain extends over the entire nuclear framework of the molecule rather than being centred about a single atom. Molecular electronic transitions, and the resulting spectra, can then be described in terms of electron transfer between two MOs. Since the nuclear framework is not rigid but is constantly undergoing vibrational motion, a convenient method of quantitatively characterizing the electronic energy of a particular MO involves the use of a potential-energy diagram whereby the potential energy of an electron in a particular MO is plotted relative to the internuclear separation in the molecule (see Figure 7—>). Molecular electronic spectra arise from the transition of an electron from one MO to another.

Energy states of real diatomic molecules
      For any real molecule, absolute separation of the different motions is seldom encountered since molecules are simultaneously undergoing rotation and vibration. The rigid-rotor, harmonic oscillator model exhibits a combined rotational-vibrational energy level satisfying EvJ = (v + 1/2)hν0 + BJ(J + 1). Chemical (chemical bonding) bonds are neither rigid nor perfect harmonic oscillators, however, and all molecules in a given collection do not possess identical rotational, vibrational, and electronic energies but will be distributed among the available energy states in accordance with the principle known as the Boltzmann distribution (Maxwell-Boltzmann distribution law).

 As a molecule undergoes vibrational motion, the bond length will oscillate about an average internuclear separation. If the oscillation is harmonic, this average value will not change as the vibrational state of the molecule changes; however, for real molecules the oscillations are anharmonic. The potential for the oscillation of a molecule is the electronic energy plotted as a function of internuclear separation (Figure 7A—>). Owing to the fact that this curve is nonparabolic, the oscillations are anharmonic and the energy levels are perturbed. This results in a decreasing energy level separation with increasing v and a modification of the vibrational selection rules to allow Δv = ±2, ±3, . . . .

      Since the moment of inertia (inertia, moment of) depends on the internuclear separation by the relationship I = μr2, each different vibrational state will possess a different value of I and therefore will exhibit a different rotational spectrum. The nonrigidity of the chemical bond in the molecule as it goes to higher rotational states leads to centrifugal distortion; in diatomic molecules this results in the stretching of the bonds, which increases the moment of inertia. The total of these effects can be expressed in the form of an expanded energy expression for the rotational-vibrational energy of the diatomic molecule; for further discussion, see the texts listed in the Bibliography (spectroscopy).

      A molecule in a given electronic state will simultaneously possess discrete amounts of rotational and vibrational energies. For a collection of molecules they will be spread out into a large number of rotational and vibrational energy states so any electronic state change (electronic transition) will be accompanied by changes in both rotational and vibrational energies in accordance with the proper selection rules. Thus any observed electronic transition will consist of a large number of closely spaced members owing to the vibrational and rotational energy changes.

Experimental methods
      There are three basic types of spectrometer systems that are commonly used for molecular spectroscopy: emission, monochromatic radiation absorption, and Fourier transform. Each of these methods involves a source of radiation, a sample, and a device for detecting and analyzing radiation.

      Emission spectrographs have some suitable means of exciting molecules to higher energy states. The radiation emitted when the molecules decay back to the original energy states is then analyzed by means of a monochromator and a suitable detector. This system is used extensively for the observation of electronic spectra. The electrons are excited to higher levels by means of an energy source such as an electric discharge or a microwave plasma. The emitted radiation generally lies in the visible or ultraviolet region. Absorption spectrometers employ as sources either broadband radiation emitters followed by a monochromator to provide a signal of very narrow frequency content or a generator that will produce a tunable single frequency. The tunable monochromatic source signal then passes through a sample contained in a suitable cell and onto a detector designed to sense the source frequency being used. The resulting spectrum is a plot of intensity of absorption versus frequency.

      A Fourier-transform spectrometer provides a conventional absorption spectrometer-type spectrum but has greater speed, resolution, and sensitivity. In this spectrometer the sample is subjected to a broadband source of radiation, resulting in the production of an interferogram due to the absorption of specific components of the radiation. This interferogram (a function of signal intensity versus time) is normally digitized, stored in computer memory, and converted to an absorption spectrum by means of a Fourier transform (see also analysis: Fourier analysis (analysis)). Fourier-transform spectrometers can be designed to cover all spectral regions from the radio-frequency to the ultraviolet.

      Spectrometers allow the study of a large variety of samples over a wide range of frequencies. Materials can be studied in the solid, liquid, or gas phase either in a pure form or in mixtures. Various designs allow the study of spectra as a function of temperature, pressure, and external magnetic and electric fields. Spectra of molecular fragments obtained by radiation of materials and of short-lived reaction intermediates are routinely observed. Two useful ways to observe spectra of short-lived species at low (4 K) temperature are to trap them in a rare gas matrix or to produce them in a pulsed adiabatic nozzle.

Fields of molecular spectroscopy
      For diatomic molecules the rotational constants for all but the very lightest ones lie in the range of 1–200 gigahertz (GHz). The frequency of a rotational transition is given approximately by ν = 2B(J + 1), and so molecular rotational spectra will exhibit absorption lines in the 2–800-gigahertz region. For polyatomic molecules three moments of inertia are required to describe the rotational motion. They produce much more complex spectra, but basic relationships, analogous to those for a diatomic molecule, exist between their moments and the observed absorption lines. The 1–1,000-gigahertz range is referred to as the microwave region (airport and police radar operate in this region) of the electromagnetic spectrum. Microwave radiation is generated by one of two methods: (1) special electronic tubes such as klystrons (klystron) or backward-wave oscillators and solid-state oscillators such as Gunn diodes, which can be stabilized to produce highly monochromatic radiation and are tunable over specific regions, and (2) frequency synthesizers, whose output is produced by the successive multiplication and addition of highly monochromatic, low-frequency signals and consists of a series of discrete frequencies with small separations that effectively provide a continuous wave signal (e.g., 6 hertz separations at 25 gigahertz).

Types of microwave spectrometer
      There are two types of microwave spectrometer in use. In the conventional Stark-modulated spectrometer, the sample is contained in a long (1- to 3-metre, or 3.3- to 9.8-foot) section of a rectangular waveguide, sealed at each end with a microwave transmitting window (e.g., mica or Mylar), and connected to a vacuum line for evacuation and sample introduction. The radiation from the source passes through a gaseous sample and is detected by a crystal diode detector that is followed by an amplifier and display system (chart recorder). In order to increase the sensitivity of the instrument, signal modulation by application of a high-voltage square wave across the sample is used. The second type is the Fourier-transform spectrometer, in which the radiation is confined in an evacuated cavity between a pair of spherical mirrors and the sample is introduced by a pulsed nozzle that lowers the temperature of the sample to less than 10 K. The sample is subjected to rotational energy excitation by application of a pulsed microwave signal, and the resulting emission signal is detected and Fourier-transformed to an absorption versus frequency spectrum. In both instruments the energy absorbed or emitted as the molecules undergo transitions from one quantized rotational state to another is observed. The Fourier-transform instrument has the advantage of providing higher resolution (1 kilohertz [kHz] relative to 30 kHz) and of exhibiting a much simpler spectrum due to the low sample temperature that insures that the majority of the molecules are in the few lowest energy states.

      For observation of its rotational spectrum, a molecule must possess a permanent electric dipole moment and have a vapour pressure such that it can be introduced into a sample cell at extremely low pressures (5–50 millitorr; one millitorr equals 1 × 10−3 millimetre of mercury or 1.93 × 10−5 pound per square inch). The spectra of molecules with structures containing up to 15 atoms can be routinely analyzed, but the density and overlapping of spectral lines in the spectra of larger molecules severely restricts analysis.

Molecular applications
      The relationship between the observed microwave transition frequency and the rotational constant of a diatomic molecule can provide a value for the internuclear distance. The quantitative geometric structures of molecules can also be obtained from the measured transitions in its microwave spectrum. In addition to geometric structures, other properties related to molecular structure can be investigated, including electric dipole moments, energy barriers to internal rotation, centrifugal distortion parameters, magnetic moments, nuclear electric quadrupole moments, vibration-rotation interaction parameters, low-frequency vibrational transitions, molecular electric quadrupole moments, and information relative to electron distribution and bonding. Microwave spectroscopy has provided the detailed structure and associated parameters for several thousand molecules.

      The use of Fourier-transform spectrometers has provided a method for studying many short-lived species such as free radicals (i.e., OH, CN, NO, CF, CCH), molecular ions (i.e., CO+, HCO+, HCS+), and Van der Waals complexes (van der Waals forces) (i.e., C6H6−HCl, H2O−H2O, Kr−HF, SO2−SO2). There is a special relationship between microwave spectroscopy and radio astronomy (radio and radar astronomy). Much of the impetus for the investigation of the microwave spectra of radical and molecular ions stems from the need for identifying the microwave emission signals emanating from extraterrestrial sources. This collaboration has resulted in the identification in outer space of several dozen species, including the hydroxyl radical, methanol, formaldehyde, ammonia, and methyl cyanide.

      For a polyatomic molecule, which is characterized by three moments of inertia (inertia, moment of), the microwave spectrum of a single molecular species provides insufficient information for making a complete structure assignment and calculating the magnitude of all bond angles and interatomic distances in the molecule. For example, the values of the three moments of inertia of the 12CH281Br12C14N molecule will depend on eight bond parameters (four angles and four distances), hence it is not possible to obtain discrete values of these eight unknowns from three moments. This problem can be circumvented by introducing the assumption that the structure of the molecule will not significantly change if one or more atoms are substituted with a different isotopic species. The three moments of an isotopically substituted molecule are then derived from its microwave spectrum and, since they depend on the same set of molecular parameters, provide three additional pieces of data from which to obtain the eight bond parameters. By determining the moments of inertia of a sufficient number of isotopically substituted species, it is possible to obtain sufficient data from which to completely determine the structure. The best structural information is obtained when an isotopic species resulting from substitution at each atom site in the molecule can be studied.

Infrared spectroscopy
      This technique covers the region of the electromagnetic spectrum between the visible (wavelength of 800 nanometres) and the short-wavelength microwave (0.3 millimetre). The spectra observed in this region are primarily associated with the internal vibrational motion of molecules, but a few light molecules will have rotational transitions lying in the region. For the infrared region, the wavenumber (wave number) (, the reciprocal of the wavelength) is commonly used to measure energy. Infrared spectroscopy historically has been divided into three regions, the near infrared (4,000–12,500 inverse centimetres [cm−1]), the mid-infrared (400–4,000 cm−1) and the far infrared (10–400 cm−1). With the development of Fourier-transform spectrometers, this distinction of areas has blurred and the more sophisticated instruments can cover from 10 to 25,000 cm−1 by an interchange of source, beam splitter, detector, and sample cell.

Infrared instrumentation
      For the near-infrared region a tungsten-filament lamp (6,000–25,000 cm−1) serves as a source. In the middle region the standard source is a Globar (50–6,000 cm−1), a silicon carbide cylinder that is electrically heated to function as a blackbody radiator. Radiation from a mercury-arc lamp (10–70 cm−1) is employed in the far-infrared region. In a grating-monochromator type instrument, the full range of the source-detector combination is scanned by mechanically changing the grating position. In a Fourier-transform instrument, the range available for a single scan is generally limited by the beam-splitter characteristics. The beam splitter functions to divide the source signal into two parts for the formation of an interference pattern. In the near-infrared region either a quartz plate or silicon deposited on a quartz plate is used. In the mid-infrared region a variety of optical-grade crystals, such as calcium flouride (CaF2), zinc selenide (ZnSe), cesium iodide (CsI), or potassium bromide (KBr), coated with silicon or germanium are employed. Below 200 cm−1 Mylar films of varying thickness are used to cover narrow portions of the region. Thermal detection of infrared radiation is based on the conversion of a temperature change, resulting from such radiation falling on a suitable material, into a measurable signal. A Golay detector employs the reflection of light from a thermally distortable reflecting film onto a photoelectric cell, while a bolometer exhibits a change in electrical resistance with a change in temperature. In both cases the device must respond to very small and very rapid changes. In the Fourier-transform spectrometers, the entire optical path can be evacuated to prevent interference from extraneous materials such as water and carbon dioxide in the air.

      A large variety of samples can be examined by use of infrared spectroscopy. Normal transmission can be used for liquids, thin films of solids, and gases. The containment of liquid and gas samples must be in a cell that has infrared-transmitting windows such as sodium chloride, potassium bromide, or cesium iodide. Solids, films, and coatings can be examined by means of several techniques that employ the reflection of radiation from the sample.

      The development of solid-state diode lasers, F-centre lasers, and spin-flip Raman lasers is providing new sources for infrared spectrometers. These sources in general are not broadband but have high intensity and are useful for the construction of instruments that are designed for specific applications in narrow frequency regions.

Analysis of absorption spectra
      The absorption of infrared radiation is due to the vibrational (vibration) motion of a molecule. For a diatomic molecule the analysis of this motion is relatively straightforward because there is only one mode of vibration, the stretching of the bond. For polyatomic molecules the situation is compounded by the simultaneous motion of many nuclei. The mechanical model employed to analyze this complex motion is one wherein the nuclei are considered to be point masses and the interatomic chemical bonds are viewed as massless springs. Although the vibrations in a molecule obey the laws of quantum mechanics, molecular systems can be analyzed using classical mechanics to ascertain the nature of the vibrational motion. Analysis shows that such a system will display a set of resonant frequencies, each of which is associated with a different combination of nuclear motions. The number of such resonances that occur is 3N − 5 for a linear molecule and 3N − 6 for a nonlinear one, where N is the number of atoms in the molecule. The motions of the individual nuclei are such that during the displacements the centre of mass of the system does not change. The frequencies at which infrared radiation is absorbed correspond to the frequencies of the normal modes of vibration or can be considered as transitions between quantized energy levels, each of which corresponds to excited states of a normal mode. An analysis of all the normal-mode frequencies of a molecule can provide a set of force constants that are related to the individual bond-stretching and bond-bending motions within the molecule.

      When examined using a high-resolution instrument and with the samples in the gas phase, the individual normal-mode absorption lines of polyatomic molecules will be separated into a series of closely spaced sharp lines. The analysis of this vibrational structure can provide the same type of information as can be obtained from rotational spectra, but even the highest resolution infrared instruments (0.0001 cm−1) cannot approach that of a Fourier-transform microwave spectrometer (10 kilohertz), and so the results are not nearly as accurate.

      Owing to the anharmonicity of the molecular vibrations, transitions corresponding to multiples (2νi, 3νi, etc, known as overtones) and combinations (ν1 + ν2, 2ν3 + ν4, etc.) of the fundamental frequencies will occur.

      The normal-mode frequencies will tend to be associated with intramolecular motions of specific molecular entities and will be found to have values lying in a relatively narrow frequency range for all molecules containing that entity. For example, all molecules containing a carboxyl group (C=O) will have a normal vibrational mode that involves the stretching of the carbon-oxygen double bond. Its particular frequency will vary, depending on the nature of the atoms or groups of atoms attached to the carbon atom but will generally occur in the region of 1,650–1,750 cm−1. This same type of behaviour is observed for other entities such as the oxygen-hydrogen (O−H) stretching motion in the hydroxyl group and the C=C stretching motion in molecules with carbon-carbon double bonds. This predictable behaviour has led to the development of spectral correlation charts that can be compared with observed infrared spectra to aid in ascertaining the presence or absence of particular molecular entities and in determining the structure of newly synthesized or unknown species. The infrared spectrum of any individual molecule is a unique fingerprint for that molecule and can serve as a reliable form of identification.

Raman spectroscopy
      Raman spectroscopy is based on the absorption of photons (photon) of a specific frequency followed by scattering at a higher or lower frequency. The modification of the scattered photons results from the incident photons either gaining energy from or losing energy to the vibrational and rotational motion of the molecule. Quantitatively, a sample (solid, liquid, or gas) is irradiated with a source frequency ν0 and the scattered radiation will be of frequency ν0 ± νi, where νi is the frequency corresponding to a vibrational or rotational transition in the molecule. Since molecules exist in a number of different rotational and vibrational states (depending on the temperature), many different values of νi are possible. Consequently, the Raman spectra will consist of a large number of scattered lines.

      Most incident photons are scattered by the sample with no change in frequency in a process known as Rayleigh scattering. To enhance the observation of the radiation at ν0 ± νi, the scattered radiation is observed perpendicular to the incident beam. To provide high-intensity incident radiation and to enable the observation of lines where νi is small (as when due to rotational changes), the source in a Raman spectrometer is a monochromatic visible laser. The scattered radiation can then be analyzed by use of a scanning optical monochromator with a phototube as a detector.

      The observation of the vibrational Raman spectrum (Raman effect) of a molecule depends on a change in the molecules polarizability (ability to be distorted by an electric field) rather than its dipole moment during the vibration of the atoms. As a result, infrared and Raman spectra provide complementary information, and between the two techniques all vibrational transitions can be observed. This combination of techniques is essential for the measurement of all the vibrational frequencies of molecules of high symmetry that do not have permanent dipole moments. Analogously, there will be a rotational Raman spectra for molecules with no permanent dipole moment that consequently have no pure rotational spectra.

Visible and ultraviolet spectroscopy

Electronic transitions
      Colours as perceived by the sense of vision are simply a human observation of the inverse of a visible absorption spectrum. The underlying phenomenon is that of an electron being raised from a low-energy molecular orbital (MO) to one of higher energy, where the energy difference is given as ΔE = hν. For a collection of molecules that are in a particular MO or electronic state, there will be a distribution among the accessible vibrational and rotational states. Any electronic transition will then be accompanied by simultaneous changes in vibrational and rotational energy states. This will result in an absorption spectrum which, when recorded under high-resolution conditions, will exhibit considerable fine structure of many closely spaced lines. Under low-resolution conditions, however, the spectrum will show the absorption of a broad band of frequencies. When the energy change is sufficiently large that the associated absorption frequency lies above 7.5 × 1014 hertz the material will be transparent to visible light and will absorb in the ultraviolet region.

  The concept of MOs can be extended successfully to molecules. For electronic transitions in the visible and ultraviolet regions only the outer (valence shell) MOs are involved. The ordering of MO energy levels as formed from the atomic orbitals (AOs) of the constituent atoms is shown in Figure 8—>. In compliance with the Pauli exclusion principle each MO can be occupied by a pair of electrons (electron) having opposite electron spins. The energy of each electron in a molecule will be influenced by the motion of all the other electrons. So that a reasonable treatment of electron energies may be developed, each electron is considered to move in an average field created by all the other electrons. Thus the energy of an electron in a particular MO is assigned. As a first approximation, the total electronic energy of the molecule is the sum of the energies of the individual electrons in the various MOs. The electronic configuration that has the lowest total energy (i.e., the ground state) will be the one with the electrons (shown as short arrows in Figure 8—>) placed doubly in the combination of orbitals having the lowest total energy. Any configuration in which an electron has been promoted to a higher energy MO is referred to as an excited state. Lying above the electron-containing MOs will be a series of MOs of increasing energy that are unoccupied. Electronic absorption transitions occur when an electron is promoted from a filled MO to one of the higher unfilled ones.

      Although the previous description of electron behaviour in molecules provides the basis for a qualitative understanding of molecular electronic spectra, it is not always quantitatively accurate. The energy calculated based on an average electric field is not equivalent to that which would be determined from instantaneous electron interactions. This difference, the electron correlation energy, can be a substantial fraction of the total energy.

Factors determining absorption regions
      The factors that determine the spectral region in which an electronic transition lies (i.e., the colour of the material) will be the energy separation between the MOs and the allowed quantum mechanical selection rules. There are certain types of molecular structures that characteristically exhibit absorptions in the visible region and others that are ultraviolet absorbers. A large class of organic compounds (organic compound), to which the majority of the dyes and inks belong, are those that contain substituted aromatic rings and conjugate multiple bonds. For example, the broad 254-nanometre transition in benzene (C6H6) can be shifted by the substitution of various organic groups for one or more of the hydrogen atoms attached to the carbon ring. The substitution of a nitroso group (NO) to give nitrosobenzene, C6H5NO, modifies the energy level spacings and shifts the absorption from the ultraviolet into the violet-blue region, yielding a compound that is pale yellow to the eye. Such shifts in spectral absorptions with substitution can be used to aid in characterizing the electron distributions in the bonds of a molecule.

      A second class of highly coloured compounds that have distinctive visible absorption are coordination compounds (coordination compound) of the transition elements (transition element) . The MOs involved in the spectral transitions for these compounds are essentially unmodified (except in energy) d-level atomic orbitals on the transition-metal atoms. An example of such a compound is the titanium (III) hydrated ion, Ti(H2O)63+, which absorbs at about 530 nanometres and appears purple to the eye.

      A large number of compounds are white solids or colourless liquids and have electronic absorption spectra only in the ultraviolet region. Inorganic salts of this type are those that contain nontransition metals and do not have any atomic d-electrons available. Covalently bonded molecules consisting of nonmetal atoms and carbon compounds with no aromatic rings or conjugated chains have all their inner orbitals fully occupied with electrons, and for the majority of them the first unoccupied MOs tend to lie at considerably higher energies than in visibly coloured compounds. Examples are sodium chloride (NaCl), calcium carbonate (CaCO3), sulfur dioxide (SO2), ethanol (ethyl alcohol) C2H5OH, and hydrocarbons (hydrocarbon) (CnHm, where n and m are integers).

      Low-resolution electronic spectra are useful as an aid in the qualitative and quantitative identification of compounds. They can serve as a fingerprint for a particular species in much the same manner as infrared spectra. Particular functional groups or molecular configurations (known as chromophores (chromophore)) tend to have strong absorptions that occur in certain regions of the visible-ultraviolet region. The precise frequency at which a particular chromophore absorbs depends significantly on the other constituents of the molecule, in general the frequency range over which its absorption is found will not be as narrow as the range of the infrared vibrational frequency associated with a specific structural entity. A strong electronic absorption band, especially in the visible region, can be used to make quantitative measurements of the concentration of the absorbing species.

      Both rotational and vibrational energies superimpose on an electronic state. This results in a very dense spectrum. The analysis of spectra of this type can provide rotational constants and vibrational frequencies for molecules not only in the ground state but also in excited states. Although the resolution is not as high as for pure rotational and vibrational spectra, it is possible to examine electronic and vibrational states whose populations are too low to be observed by these methods. Improvements in resolution of electronic spectra can be achieved by the use of laser sources (see below Laser spectroscopy (spectroscopy)).

Fluorescence and phosphorescence
      These phenomena are closely related to electronic absorption spectra and can be used as a tool for analysis and structure determination. Both involve the absorption of radiation via an electronic transition, a loss of energy through either vibrational energy decay or nonradiative processes, and the subsequent emission of radiation of a lower frequency than that absorbed.

      Electrons possess intrinsic magnetic moments that are related to their spin angular momenta. The spin quantum number is s = 1/2, so in the presence of a magnetic field an electron can have one of two orientations corresponding to magnetic spin quantum number ms = ±1/2. The Pauli exclusion principle requires that no two electrons in an atom have the same identical set of quantum numbers; hence when two electrons reside in a single AO or MO they must have different ms values (i.e., they are antiparallel, or spin paired). This results in a cancellation of their magnetic moments, producing a so-called singlet state. Nearly all molecules that contain an even number of electrons have singlet ground states and have no net magnetic moment (such species are called diamagnetic (diamagnetism)). When an electron absorbs energy and is excited to a higher energy level, there exists the possibility of (1) retaining its antiparallel configuration relative to the other electron in the orbital from which it was promoted so that the molecule retains its singlet characteristic, or (2) changing to a configuration in which its magnetic moment is parallel to that of its original paired electron. In the latter case, the molecule will possess a net magnetic moment (becoming paramagnetic (paramagnetism)) and is said to be in a triplet state. For each excited electronic state, either electron spin configuration is possible so that there will be two sets of energy levels (see Figure 9). The normal selection rules forbid transitions between singlet (Si) and triplet (Ti) states; hence there will be two sets of electronic transitions, each associated with one of the two sets of energy levels.

      Fluorescence is the process whereby a molecule in the lower of two electronic states (generally the ground state) is excited to a higher electronic state by radiation whose energy corresponds to an allowed absorption transition, followed by the emission of radiation as the system decays back to the original state. The decay process can follow several pathways. If the decay is back to the original lower state, the process is called resonance fluorescence and occurs rapidly, in about one nanosecond. Resonance fluorescence is generally observed for monatomic gases and for many organic molecules, in particular aromatic systems that absorb in the visible and near-ultraviolet regions. For many molecules, especially aromatic compounds (aromatic compound) whose electronic absorption spectra lie predominately in the shorter-wavelength ultraviolet region (below 400 nanometres), the lifetime of the excited electronic state is sufficiently long that prior to the emission of radiation the molecule can (1) undergo a series of vibrational state decays, (2) lose energy through interstate transfer (intersystem crossing), or (3) lose vibrational energy via molecular collisions.

 In the first case, the system will emit radiation in the infrared region as the vibrational energy of the excited state decays back to the lowest vibrational level. The molecule then undergoes an electronic state decay back to one of the vibrational states associated with the lower electronic state. The resulting emission spectrum will then be centred at a frequency lower than the absorption frequency and will appear to be a near mirror image of the absorption spectrum. The second mechanism can be illustrated by reference to the potential energy curves for nitrogen hydride (NH) shown in Figure 7B—>. The curves for the 1Σ+ and 1Π states intersect at a radius value of 0.2 nanometre. If a molecule in the 1Π excited electronic state is in a vibrational level corresponding to the energy value of this intersection point, it can cross over to the 1Σ+ state without emission or absorption of radiation. Subsequently it can undergo vibrational energy loss to end up in the lowest vibrational state of the 1Σ+ electronic state. This can then be followed by an electronic transition back to the lower 1Δ state. Thus the absorption of energy corresponding to an original 1Δ → 1Π transition results in the emission of fluorescence radiation corresponding to the lower frequency 1Σ+1Δ transition. In the third case, when two molecules collide (collision) there exists the possibility for energy transfer between them. Upon colliding, a molecule can thus be transformed into a different electronic state whose energy minimum may lie lower or higher than its previous electronic state.

      The lifetimes of the excited singlet electronic states, although long enough to allow vibrational relaxation or intersystem crossing, are quite short, so that fluorescence occurs on a time scale of milliseconds to microseconds following irradiation of a material. The most common mode of observation of fluorescence is that of using ultraviolet radiation (invisible to the human eye) as an exciting source and observing the emission of visible radiation. In addition to its use as a tool for analysis and structural determination of molecules, there are many applications outside the laboratory. For example, postage stamps may be tagged with a visually transparent coating of a fluorescing agent to prevent counterfeiting, and the addition of a fluorescing agent with emissions in the blue region of the spectrum to detergents will impart to cloth a whiter appearance in the sunlight.

      Phosphorescence is related to fluorescence in terms of its general mechanism but involves a slower decay. It occurs when a molecule whose normal ground state is a singlet is excited to a higher singlet state, goes to a vibrationally excited triplet state via either an intersystem crossing or a molecular collision, and subsequently, following vibrational relaxation, decays back to the singlet ground state by means of a forbidden transition. The result is the occurrence of a long lifetime for the excited triplet state; several seconds up to several hours are not uncommon. These long lifetimes can be related to interactions between the intrinsic (spin) magnetic moments of the electrons and magnetic moments resulting from the orbital motion of the electrons.

      Molecules in singlet and triplet states react chemically in different manners. It is possible to affect chemical reactions by the transfer of electronic energy from one molecule to another in the reacting system. Thus the study of fluorescence and phosphorescence provides information related to chemical reactivity.

Photoelectron spectroscopy
      Photoelectron spectroscopy is an extension of the photoelectric effect (see radiation: The photoelectric effect. (radiation)), first explained by Einstein in 1905, to atoms and molecules in all energy states. The technique involves the bombardment of a sample with radiation from a high-energy monochromatic source and the subsequent determination of the kinetic energies of the ejected electrons. The source energy, hν, is related to the energy of the ejected electrons, (1/2)mev2, where me is the electron mass and v is the electron velocity, by hν = (1/2)mev2 + Φ, where Φ is the ionization energy of the electron in a particular AO or MO. When the energy of the bombarding radiation exceeds the ionization energy, the excess energy will be imparted to the ejected electron in the form of kinetic energy. By knowing the source frequency and measuring the kinetic energies of the ejected electrons, the ionization energy of an electron in each of the AOs or MOs of a system can be determined. This method serves to complement the data obtained from electronic absorption spectra and in some cases provides information that cannot be obtained from electronic spectroscopy because of selection rules.

Laser spectroscopy
      As mentioned above, the invention and subsequent development of the laser opened many new areas of spectroscopy. Although the basic processes investigated remain those of rotational, vibrational, and electronic spectroscopies, this tool has provided many new ways to investigate such phenomena and has allowed the acquisition of data previously unavailable. At least two dozen new types of experiments using lasers have been developed. To illustrate the nature and utility of lasers in spectroscopy a limited number will be reviewed.

      Lasers by their nature provide an output that consists of a relatively small number of very narrow-banded transitions. While these high-intensity sources can provide radiation useful for certain limited types of spectroscopic studies, a high-intensity tunable narrow-band source is needed for conventional high-resolution spectroscopic studies. This type of source is provided by the dye laser, in which laser emissions arise from the decay of dye molecules that have been excited into a multitude of closely spaced rovibronic (rotational-vibrational-electronic) levels by the application of an intense secondary laser signal (a process known as pumping (optical pumping)). Dye lasers can provide radiation over a limited region within the range of 330 to 1,250 nanometres. The region covered by the radiation can be varied by changing the dye and pump source. Thus there exist essentially continuously tunable sources in the region where electronic spectra are normally observed. Although lasers with continuous tunability over all spectral ranges of interest are not available, it is possible to observe transitions between molecular energy levels by using a fixed-frequency laser and shifting the energy levels by application of electric or magnetic fields to the sample. Other techniques such as the observation of fluorescence, dissociation, multiple photon absorption, and double resonance are used to enhance sensitivity and circumvent the lack of tunability. While the use of conventional spectroscopic methods generally employs established designs of spectrometers and techniques, the use of lasers often requires the development of new and ingenious experimental methods to extract desired spectroscopic information.

Doppler-limited spectroscopy
 With the exception of specially designed molecular-beam spectrometers, the line width of a molecular absorption transition is limited by the Doppler effect. The resolution of conventional spectrometers, with the exception of a few very expensive Fourier-transform instruments, is generally limited to a level such that observed line widths are well in excess of the Doppler width. Tunable laser sources with extremely narrow bandwidths and high intensity routinely achieve a resolution on the order of the Doppler line width (0.001–0.05 nanometre). The design of a laser absorption spectrometer (Figure 10—>) is advantageous in that no monochromator is needed since the absorption coefficient of a transition can be measured directly from the difference in the photodiode current generated by the radiation beam passing through the sample (I1) and the current generated by a reference beam (I2). In addition, the high power available from laser sources, concurrent with their frequency and intensity stabilization, eliminates problems with detector noise. Since the sensitivity of detecting spectral transitions increases with resolution, laser spectrometers are inherently more sensitive than conventional broadband source types. The extremely narrow nature of a laser beam permits it to undergo multiple reflections through a sample without spatial spreading and interference, thus providing long absorption path lengths. Lasers can be highly frequency-stabilized and accurately measured, one part in 108 being routinely achieved. A small fraction of the source signal can be diverted to an interferometer and a series of frequency markers generated and placed on the recording of the spectral absorption lines. Lasers can be tuned over a range of several wavenumbers in a time scale of microseconds, making laser spectrometers ideal instruments for detecting and characterizing short-lived intermediate species in chemical reactions. Laser spectrometers offer two distinct advantages for the study of fluorescence and phosphorescence. The high source intensity enables the generation of larger upper-state populations in the fluorescencing species. The narrow frequency band of the source provides for greater energy selectivity of the upper state that is being populated.

Coherent anti-Stokes Raman spectroscopy (CARS)
      This technique involves the phenomenon of wave mixing, takes advantage of the high intensity of stimulated Raman scattering, and has the applicability of conventional Raman spectroscopy. In the CARS method two strong collinear laser beams at frequencies ν1 and ν21 > ν2) irradiate a sample. If the frequency difference, ν1 − ν2, is equal to the frequency of a Raman-active rotational or vibrational transition νR, then the efficiency of wave mixing is enhanced and signals at νA = 2ν1 − ν2 (anti-Stokes) and νS = 2ν2 − ν1 (Stokes (Stokes lines)) are produced by wave mixing due to the nonlinear polarization of the medium. While either output signal may be detected, the anti-Stokes frequency is well above ν1 and has the advantage of being readily separated by optical filtering from the incident beams and fluorescence that may be simultaneously generated in the sample. Although the same spectroscopic transitions, namely, those with frequencies νR, are determined from both conventional Raman spectroscopy and CARS, the latter produces signals that have intensities 104–105 times as great. This enhanced signal level can greatly reduce the time necessary to record a spectrum. Owing to the coherence of the generated signals, the divergence of the output beam is small, and good spatial discrimination against background signals is obtained. Such noise may occur in the examination of molecules undergoing chemiluminescence or existing in either flames or electric discharges. Since the generation of the anti-Stokes signal occurs in a small volume where the two incident beams are focused, sample size does not have to be large. Microlitre-size liquid samples and gases at millitorr pressures can be used. Another advantage of the spatial discrimination available is the ability to examine different regions within a sample. For example, CARS can be used to determine the composition and local temperatures in flames and plasmas. Owing to the near collinearity of the exciting and observing signals, the Doppler effect is minimized and resolution of 0.001 cm−1 can be achieved. The primary disadvantage of the technique is the need for laser sources with excellent intensity stabilization.

Laser magnetic resonance and Stark spectroscopies
      Because of the nature of laser-signal generation, most lasers are not tunable over an appreciable frequency range and even those that can be tuned, such as dye lasers, must be driven by a pump laser and for a given dye have a limited tuning range. This limitation can be overcome for molecules that possess permanent magnetic moments or electric dipole moments by using external magnetic (magnetic field) or electric fields (electric field) to bring the energy spacing between levels into coincidence with the frequency of the laser.

 Molecules that have one or more unpaired electrons will possess permanent magnetic moments. Examples of such paramagnetic (paramagnetism) systems are free radicals (radical) such as NO, OH, and CH2 and transition-metal ions like Fe(H2O)63+ and Cr(CN)64−. A hypothetical electronic energy-level diagram for a radical having a single unpaired electron and two energy levels, a ground state having zero orbital angular momentum (L = 0), and an excited state with L = 1 is shown in Figure 11—>. When the magnetic field is increased, the separation of the Zeeman (Zeeman effect) components will shift, and each allowed transition (ΔM = 0 or ±1, where M = L + MS [spin angular momentum]) will progressively come into coincidence with the laser frequency and a change in signal intensity will be observed. To enhance the sensitivity of this technique, the sample is often placed inside the laser cavity, and the magnetic field is modulated. By making the laser cavity part of a reacting flow system, the presence of paramagnetic reaction intermediates can be detected and their spectra recorded. Concentrations of paramagnetic species as low as 109 molecules per cubic centimetre have been observed. This method has made it possible to identify radicals observed in interstellar space and to provide spectral detail for them.

 An analogous method, called Stark spectroscopy, involves the use of a strong variable electric field to split and vary the spacing of the energy levels of molecules that possess a permanent electric dipole moment. The general principle is embodied in Figure 11—>, with the substitution of an electric field for the magnetic field. Since very high fields (1,000–5,000 volts per centimetre) are required, the sample must be located between closely spaced metal plates. This precludes the inclusion of the sample inside the laser cavity. Sensitivity is enhanced by modulating the electric field. Although the frequency of the laser can be stabilized and measured to within 20–40 kilohertz, the determination of molecular parameters is limited to the accuracy inherent in the measurement of the electric field—namely, one part in 104. This method is useful for the determination of the dipole moment and structure of species whose rotational transitions fall above the microwave region.

Jack D. Graybeal

X-ray and radio-frequency spectroscopy

X-ray spectroscopy
      A penetrating, electrically uncharged radiation was discovered in 1895 by the German physicist Wilhelm Conrad Röntgen (Röntgen, Wilhelm Conrad) and was named X-radiation (X-ray) because its origin was unknown. This radiation is produced when electrons (cathode rays (cathode ray)) strike glass or metal surfaces in high-voltage evacuated tubes and is detected by the fluorescent glow of coated screens and by the exposure of photographic plates and films. The medical applications (medicine) of such radiation that can penetrate flesh more easily than bone were recognized immediately, and X rays were being used for medical purposes in Vienna within three months of their discovery. Over the next several years, a number of researchers determined that the rays carried no electric charge, traveled in straight trajectories, and had a transverse nature (could be polarized) by scattering from certain materials. These properties suggested that the rays were another form of electromagnetic radiation, a possibility that was postulated earlier by the British physicist J.J. Thomson (Thomson, Sir J.J.). He noted that the electrons (electron) that hit the glass wall of the tube would undergo violent accelerations (acceleration) as they slowed down, and, according to classical electromagnetism, these accelerations would cause electromagnetic radiation to be produced.

      The first clear demonstration of the wave nature of X rays was provided in 1912 when they were diffracted (X-ray diffraction) by the closely spaced atomic planes in a crystal of zinc sulfide. Because the details of the diffraction patterns depended on the wavelength of the radiation, these experiments formed the basis for the spectroscopy of X rays. The first spectrographs for this radiation were devised in 1912–13 by two British physicists—father and son—William Henry (Bragg, Sir William) and Lawrence Bragg (Bragg, Sir Lawrence), who showed that there existed not only continuum X-ray spectra, to be expected from processes involving the stopping of charged particles in motion, but also discrete characteristic spectra (each line resulting from the emission of a definite energy), indicating that some X-ray properties are determined by atomic structure. The systematic increase of characteristic X-ray energies with atomic number was shown by the British physicist Henry G.J. Moseley (Moseley, Henry Gwyn Jeffreys) in 1913 to be explainable on the basis of the Bohr theory of atomic structure, but more quantitative agreement between experiment and theory had to await the development of quantum mechanics. Wavelengths (wavelength) for X rays range from about 0.1 to 200 angstroms, with the range 20 to 200 angstroms known as soft X rays.

Relation to atomic (atom) structure
      X rays can be produced by isolated atoms and ions (ion) by two related processes. If two or more electrons are removed from an atom, the remaining outer electrons are more tightly bound to the nucleus by its unbalanced charge, and transitions of these electrons from one level to another can result in the emission of high-energy photons (photon) with wavelengths of 100 angstroms or less. An alternate process occurs when an electron in a neutral atom is removed from an inner shell. This removal can be accomplished by bombarding the atom with electrons, protons, or other particles at sufficiently high energy and also by irradiation of the atom by sufficiently energetic X rays. The remaining electrons in the atom readjust very quickly, making transitions to fill the vacancy left by the removed electron, and X-ray photons are emitted in these transitions. The latter process occurs in an ordinary X-ray tube, and the resultant series of X-ray lines, the characteristic spectrum, is superimposed on a spectrum of continuous radiation resulting from accelerated electrons.

      The shells (shell atomic model) in an atom, designated as n = 1, 2, 3, 4, 5 by optical spectroscopists, are labeled K, L, M, N, O . . . by X-ray spectroscopists. If an electron is removed from a particular shell, electrons from all the higher-energy shells can fill that vacancy, resulting in a series that appears inverted as compared with the hydrogen series. Also, the different angular momentum states for a given shell cause energy sublevels within each shell; these subshells are labeled by Roman numerals according to their energies.

      The X-ray fluorescence radiation of materials is of considerable practical interest. Atoms irradiated by X rays having sufficient energies, either characteristic or continuous rays, lose electrons and as a result emit X rays characteristic of their own structures. Such methods are used in the analyses of mixtures of unknown composition.

      Sometimes an electron with a definite energy is emitted by the atom instead of an X-ray photon when electrons in the outer shells cascade to lower energy states. This process is known as Auger emission (Auger effect). Auger spectroscopy, the analysis of the energy of the emitted electrons when a surface is bombarded by electrons at a few kilovolt energies, is commonly used in surface science to identify the elemental composition of the surface.

 If the continuous spectrum from an X-ray source is passed through an absorbing material, it is found that the absorption coefficient changes sharply at X-ray wavelengths corresponding to the energy just required to remove an electron from a specific inner shell to form an ion. The sudden increase of the absorption coefficient as the wavelength is reduced past the shell energy is called an absorption edge; there is an absorption edge associated with each of the inner shells. They are due to the fact that an electron in a particular shell can be excited above the ionization energy of the atom. The X-ray absorption cross section for photon energies capable of ionizing the inner-shell electrons of lead is shown in Figure 12—>. X-ray absorption edges are useful for determining the elemental composition of solids or liquids (see below Applications (spectroscopy)).

Production methods

X-ray tubes (X-ray tube)
      The traditional method of producing X rays is based on the bombardment of high-energy electrons on a metal target in a vacuum tube. A typical X-ray tube consists of a cathode (a source of electrons, usually a heated filament) and an anode, which are mounted within an evacuated chamber or envelope. A potential difference of 10–100 kilovolts is maintained between cathode (the negative electrode) and anode (the positive electrode). The X-ray spectrum emitted by the anode consists of line emission and a continuous spectrum of radiation called bremsstrahlung radiation. The continuous spectrum results from the violent deceleration of charges (the sudden “braking”) of the electrons as they hit the anode. The line emission is due to outer shell electrons falling into inner shell vacancies and hence is determined by the material used to construct the anode. The shortest discrete wavelengths are produced by materials having the highest atomic numbers.

Synchrotron sources
      Electromagnetic radiation is emitted by all accelerating charged particles. For electrons moving fairly slowly in a circular orbit, the emission occurs in a dipole radiation pattern highly peaked at the orbiting frequency. If the electrons are made to circulate at highly relativistic speeds (i.e., those near the speed of light, where the kinetic energy of each electron is much higher than the electron rest mass energy), the radiation pattern collapses into a forward beam directed tangent to the orbit and in the direction of the moving electrons. This so-called synchrotron radiation, named after the type of accelerator where this type of radiation was first observed, is continuous and depends on the energy and radius of curvature of the ring; the higher the acceleration, the higher is the energy spectrum.

      The typical synchrotron source consists of a linear electron accelerator that injects high-energy electrons into a storage ring (colliding-beam storage ring) (see particle accelerator: Synchrotrons (particle accelerator)). Since the intensity of the synchrotron radiation is proportional to the circulating current, many electron pulses from the injecting accelerator are packed into a single high-current bunch of electrons, and many separate bunches can be made to circulate simultaneously in the storage ring. The radiation can be made even more intense by passing the high-energy electrons (typically a few billion electron volts in energy) through a series of wiggler or undulator magnets that cause the electrons to oscillate or spiral rapidly.

      The high intensity and broad tunability of synchrotron sources has had enormous impact on the field of X-ray physics. The brightness of synchrotron X-ray sources (brightness is defined as the amount of power within a given small energy band, cross section area of the source, and divergence of the radiation) is more than 10 orders of magnitude higher than the most powerful rotating anode X-ray machines. The synchrotron sources can also be optimized for the vacuum-ultraviolet portion, the soft (low-energy) X-ray portion (between 20 and 200 angstroms), or the hard (high-energy) X-ray portion (1–20 angstroms) of the electromagnetic spectrum.

X-ray optics
      X rays are strongly absorbed by solid matter so that the optics used in the visible and near-infrared portions of the electromagnetic spectrum cannot be used to focus or reflect the radiation. Over a fairly wide range of X-ray energies, however, radiation hitting a metal surface at grazing incidence can be reflected (reflection). For X rays where the wavelengths are comparable to the lattice spacings in analyzing crystals, the radiation can be “Bragg reflected” from the crystal: each crystal plane acts as a weakly reflecting surface, but if the angle of incidence θ and crystal spacing d satisfy the Bragg condition (Bragg law), 2d sin θ = nλ, where λ is the wavelength of the X ray and n is an integer called the order of diffraction, many weak reflections can add constructively to produce nearly 100 percent reflection. The Bragg condition for the reflection of X rays is similar to the condition for optical reflection from a diffraction grating. Constructive interference occurs when the path difference between successive crystal planes is equal to an integral number of wavelengths of the electromagnetic radiation.

      X-ray monochromators are analogous to grating monochromators and spectrometers in the visible portion of the spectrum. If the lattice spacing for a crystal is accurately known, the observed angles of diffraction can be used to measure and identify unknown X-ray wavelengths. Because of the sensitive wavelength dependence of Bragg reflection exhibited by materials such as silicon, a small portion of a continuous spectrum of radiation can be isolated. Bent single crystals used in X-ray spectroscopy are analogous to the curved line gratings used in optical spectroscopy. The bandwidth of the radiation after it has passed through a high-resolution monochromator can be as narrow as Δλ/λ = 10−4, and, by tilting a pair of crystals with respect to the incident radiation, the wavelength of the diffracted radiation can be continuously tuned without changing the direction of the selected light.

      For X-ray wavelengths significantly longer than the lattice spacings of crystals, “superlattices” consisting of alternating layers of atoms with high and low atomic numbers can be made to reflect the softer X rays. It is possible to construct these materials where each layer thickness (a layer may consist of hundreds of atoms to a single atom) can be controlled with great precision. Normal-incidence mirrors with more than 40 percent efficiency in the soft X-ray portion of the spectrum have been made using this technology.

X-ray detectors
      The first X-ray detector used was photographic film; it was found that silver halide crystallites would darken when exposed to X-ray radiation. Alkali halide crystals such as sodium iodide combined with about 0.1 percent thallium have been found to emit light when X rays are absorbed in the material. These devices are known as scintillators, and when used in conjunction with a photomultiplier tube they can easily detect the burst of light from a single X-ray photon. Furthermore, the amount of light emitted is proportional to the energy of the photon, so that the detector can also be used as a crude X-ray spectrometer. The energy resolution of sodium iodide is on the order of 10 percent of the total energy deposited in the crystal. X-ray photons are readily absorbed by the material; the mean distance that a 0.5-million electron volt (MeV) photon will travel before being absorbed is three centimetres.

      Semiconductor (semiconductor device) crystals such as silicon or germanium are used as X-ray detectors in the range from 1,000 electron volts (1 keV) to more than 1 MeV. An X-ray photon absorbed by the material excites a number of electrons from its valence band to the conduction band. The electrons in the conduction band and the holes in the valence band are collected and measured, with the amount of charge collected being proportional to the energy of the X-ray photon. Extremely pure germanium crystals have an energy resolution of 1 keV and an X-ray energy of 1 MeV.

      Low-temperature bolometers (bolometer) are also used as high-resolution X-ray detectors. X rays absorbed in semiconductors and cooled to very low temperatures (approximately 0.1 K or less) deposit a small amount of heat. Because the material has a low heat capacity at those temperatures, there is a measurable rise in temperature. Energy resolution as high as 1 eV out of 10 keV X rays have been obtained.

      X rays also can be detected by an ionization chamber consisting of a gas-filled container with an anode and a cathode. When an X-ray photon enters the chamber through a thin window, it ionizes the gas inside, and an ion current is established between the two electrodes. The gas is chosen to absorb strongly in the desired wavelength region. With increased voltage applied across the electrodes, the ionization chamber becomes a proportional counter, which produces an amplified electrical pulse when an X-ray photon is absorbed within it. At still higher voltages, absorption of an X-ray photon with consequent ionization of many atoms in the gas initiates a discharge breakdown of the gas and causes a large electric pulse output. This device is known as a Geiger-Müller tube, and it forms the basis for radiation detectors known as Geiger counters (see radiation measurement: Active detectors: Gas-filled detectors: Geiger-Müller counters (radiation measurement)).

 The earliest application of X rays was medical (medicine): high-density objects such as bones would cast shadows on film that measured the transmission of the X rays through the human body. With the injection of a contrast fluid that contains heavy atoms such as iodine, soft tissue also can be brought into contrast. Synchronized flash X-ray photography, made possible with the intense X rays from a synchrotron source, is shown in Figure 13—>. The photograph has captured the image of pulsing arteries of the human heart that would have given a blurred image with a conventional X-ray exposure.

      A source of X rays of known wavelength also can be used to find the lattice spacing, crystal orientation, and crystal structure of an unknown crystalline material. The crystalline material is placed in a well-collimated beam of X rays, and the angles of diffraction (X-ray diffraction) are recorded as a series of spots on photographic film. This method, known as the Laue method (Laue diffraction pattern) (after the German physicist Max Theodor Felix von Laue), has been used to determine and accurately measure the physical structure of many materials, including metals and semiconductors. For more complex structures such as biological molecules, thousands of diffraction spots can be observed, and it is a nontrivial task to unravel the physical structure from the diffraction patterns. The atomic structures of deoxyribonucleic acid (DNA) and hemoglobin were determined through X-ray crystallography. X-ray scattering is also employed to determine near-neighbour distances of atoms in liquids and amorphous solids.

      X-ray fluorescence and location of absorption edges (absorption edge) can be used to identify quantitatively the elements (chemical element) present in a sample. The innermost core-electron energy levels are not strongly perturbed by the chemical environment of the atom since the electric fields acting on these electrons are completely dominated by the nuclear charge. Thus, regardless of the atom's environment, the X-ray spectra of these electrons have nearly the same energy levels as they would if the atom were in a dilute gas; their atomic energy level fingerprint is not perturbed by the more complex environment. The elemental abundance of a particular element can be determined by measuring the difference in the X-ray absorption just above and just below an absorption edge of that element. Furthermore, if optics are used to focus the X rays onto a small spot on the sample, the spatial location of a particular element can be obtained.

      Just above the absorption edge of an element, small oscillations in the absorption coefficient are observed when the incident X-ray energy is varied. In extended X-ray absorption fine structure spectroscopy (EXAFS), interference effects generated by near neighbours of an atom that has absorbed an X ray, and the resulting oscillation frequencies, are analyzed so that distances to the near-neighbour atoms can be accurately determined. The technique is sensitive enough to measure the distance between a single layer of atoms adsorbed on a surface and the underlying substrate.

      Emission of X rays from high-temperature laboratory plasmas (plasma) is used to probe the conditions within them; X-ray spectral measurements show both the composition and temperature of a source. X-ray and gamma-ray (gamma ray) astrophysics is also an active area of research. X-ray sources (X-ray source) include stars and galactic centres. The most intense astronomical X-ray sources are extremely dense gravitational objects such as neutron stars (neutron star) and black holes (black hole). Matter falling toward these objects is heated to temperatures as high as 1010 K, resulting in X-ray and soft gamma-ray emissions. Because X rays are absorbed by the Earth's atmosphere, such measurements are made above the atmosphere by apparatus carried by balloons, rockets, or orbiting satellites.

Radio-frequency spectroscopy
      The energy states (energy state) of atoms (atom), ions (ion), molecules, and other particles are determined primarily by the mutual attraction of the electrons (electron) and the nucleus and by the mutual repulsion of the electrons. Electrons and nuclei have magnetic (magnetism) properties in addition to these electrostatic properties. The spin-orbit interaction has been discussed above (see Foundations of atomic spectra: Hydrogen atom states: Fine and hyperfine structure of spectra (spectroscopy)). Other, usually weaker, magnetic interactions within the atom exist between the magnetic moments of different electrons and between the magnetic moment of each electron and the orbital motions of others. Energy differences between levels having different energies owing to magnetic interactions vary from less than 107 hertz to more than 1013 hertz, being generally greater for heavy atoms.

      Nuclei of atoms often have intrinsic angular momentum ( spin) and magnetic moments because of the motions and intrinsic magnetic moments of their constituents, and the interactions of nuclei with the magnetic fields (magnetic field) of the circulating electrons affect the electron energy states. As a result, an atomic level that consists of several states having the same energy when the nucleus is nonmagnetic may be split into several closely spaced levels when the nucleus has a magnetic moment. The levels will have different energies, depending on the relative orientation of the nucleus and the magnetic field produced by the surrounding electrons. This additional structure of an atom's levels or of spectral lines caused by the magnetic properties of its nucleus is called magnetic hyperfine structure. Separations between levels differing only in the relative orientation of the magnetic field of the nucleus and electron range typically from 106 hertz to 1010 hertz.

      Atoms, ions, and molecules can make transitions (transition) from one state to another state that differs in energy because of one or more of these magnetic effects. Molecules also undergo transitions between rotational and vibrational states. Such transitions either can be spontaneous or can be induced by the application of appropriate external electromagnetic fields at the resonant frequencies. Transitions also can occur in atoms, molecules, and ions between high-energy electronic states near the ionization limit. The resulting spectra are known as radio-frequency (rf) spectra, or microwave spectra; they are observed typically in the frequency range from 106 to 1011 hertz.

      The spontaneous transition rate as an atom goes from an excited level to a lower one varies roughly as the cube of the frequency of the transition. Thus, radio-frequency and microwave transitions occur spontaneously much less rapidly than do transitions at visible and ultraviolet frequencies. As a result, most radio-frequency and microwave spectroscopy is done by forcing a sample of atoms to absorb radiation instead of waiting for it to emit radiation spontaneously. These methods are facilitated by the availability of powerful electronic oscillators throughout this frequency range. The principal exception occurs in the field of radio astronomy (radio and radar astronomy); the number of atoms or ions in an astronomical source is large enough so that spontaneous emission spectra may be collected by large antennas and then amplified and detected by cooled, low-noise electronic devices.

      The first measurements of the absorption spectra of molecules for the purpose of finding magnetic moments were made in the late 1930s by an American physicist, Isidor Rabi (Rabi, Isidor Isaac), and his collaborators, using molecular (molecular beam) and atomic beams. A beam focused by magnets in the absence of a radio-frequency field was defocused and lost when atoms were induced to make transitions to other states. The radio-frequency or microwave spectrum was taken by measuring the number of atoms that remained focused in the apparatus while the frequency was varied. One of the most famous laboratory experiments with radio-frequency spectra was performed in 1947 by two American physicists, Willis Lamb (Lamb, Willis Eugene, Jr.) and Robert Retherford. Their experiment measured the energy difference between two nearly coincident levels in hydrogen, designated as 22S1/2 and 22P1/2. Although optical measurements had indicated that these levels might differ in energy, the measurements were complex and were open to alternative interpretations. Atomic theory at the time predicted that those levels should have identical energies. Lamb and Retherford showed that the energy levels were in fact separated by about 1,058 megahertz; hence the theory was incomplete. This energy separation in hydrogen, known as the Lamb shift, contributed to the development of quantum electrodynamics.

      Radio-frequency measurements of energy intervals in ground levels and excited levels of atoms can be made by placing a sample of atoms (usually a vapour in a glass cell) within the coil of an oscillator and tuning the device until a change is seen in the absorption of energy from the oscillator by the atoms. In the method known as optical double resonance, optical radiation corresponding to a transition in the atom of interest is passed through the cell. If radio-frequency radiation is absorbed by the atoms in either of the levels involved, the intensity, polarization, or direction of the fluorescent light may be changed. In this way a sensitive optical measurement indicates whether or not a radio-frequency interval in the atom matches the frequency applied by the oscillator.

      Microwave amplification by stimulated emission of radiation (the maser) was invented by an American physicist, Charles Townes (Townes, Charles Hard), and two Russian physicists, Nikolai Basov (Basov, Nikolay Gennadiyevich) and Alexandr Prokhorov (Prokhorov, Aleksandr Mikhaylovich), in 1951 and 1952, and stimulated the invention of the laser. If atoms are placed in a cavity tuned to the transition between two atomic levels such that there are more atoms in the excited state than in the ground state, they can be induced to transfer their excess energy into the electromagnetic radiation resonant in the cavity. This radiation, in turn, stimulates more atoms in the excited state to emit radiation. Thus an oscillator is formed that resonates at the atomic frequency.

      Microwave frequencies between atomic states can be measured with extraordinary precision. The energy difference between the hyperfine levels of the ground state in the cesium atom (atomic time) is currently the standard time interval. One atomic second is defined as the time it takes for the cesium frequency to oscillate 9,192,631,770 times. Such atomic clocks (atomic clock) have a longer-term uncertainty in their frequency that is less than one part in 1013. Measurement of time intervals based on the cesium atom's oscillations are more accurate than those based on Earth rotation since friction caused by the tides and the atmosphere is slowing down the rotation rate (i.e., our days and nights are becoming slightly longer). Since an international time scale based on an atomic-clock time standard has been established, “leap seconds” must be periodically introduced to the scale known as Coordinated Universal Time (UTC) to keep the “days” in synchronism with the more accurate atomic clocks.

      In those atoms in which the nucleus has a magnetic moment, the energies of the electrons depend slightly on the orientation of the nucleus relative to the magnetic field produced by the electrons near the centre of the atom. The magnetic field at the nucleus depends somewhat on the environment in which the atom is found, which in turn depends on the neighbouring atoms. Thus the radio-frequency spectrum of a substance's nuclear magnetic moments reflects both the constituents and the forms of chemical binding in the substance. Spectra resulting when the orientation of the nucleus is made to oscillate by a time-varying magnetic field are known as nuclear magnetic-resonance (NMR) spectra and are of considerable utility in identification of organic compounds. The first nuclear magnetic resonance experiments were published independently in 1946 by two American physicists, Edward Purcell (Purcell, E.M.) and Felix Bloch (Bloch, Felix). A powerful medical (medicine) application of NMR spectroscopy, magnetic resonance imaging, is used to allow visualization of soft tissue in the human body. This technique is accomplished by measuring the NMR (nuclear magnetic resonance) signal in a magnetic field that varies in each of the three dimensions. Through the use of pulse techniques, the NMR signal strength of the proton (hydrogen) resonance as a function of the resonance frequency can be obtained, and a three-dimensional image of the proton-resonance signal can be constructed. Because body tissue at different locations will have a different resonance frequency, three-dimensional images of the body can be produced.

      Radio-frequency transitions have been observed in astronomy. Observation of the 21-centimetre (1,420-megahertz) transition between the hyperfine levels in the ground level of hydrogen have provided much information about the temperature and density of hydrogen clouds in the Sun's galaxy, the Milky Way Galaxy. Charged particles spiraling in galactic magnetic fields emit synchrotron radiation in the radio and microwave regions. Intergalactic molecules and radicals have been identified in radio-astronomy spectroscopy, and naturally occurring masers have been observed. The three-degree blackbody spectrum that is the remnant of the big bang creation of the universe (see above) covers the microwave and far-infrared portion of the electromagnetic spectrum. Rotating neutron stars (neutron star) that emit a narrow beam of radio-frequency radiation (much like the rotating beam of a lighthouse) are observed through the reception of highly periodic pulses of radio-frequency radiation. These pulsars (pulsar) have been used as galactic clocks to study other phenomena. By studying the spin-down rate of a pulsar in close orbit with a companion star, Joseph Taylor (Taylor, Joseph H., Jr.), an American astrophysicist, was able to show that a significant amount of the rotational energy lost was due to the emission of gravitational (gravitation) radiation. The existence of gravitational radiation is predicted by Einstein's general theory of relativity but has not yet been seen directly.

John Oliver Stoner, Jr. Steven Chu

Resonance-ionization spectroscopy
      Resonance-ionization spectroscopy (RIS) is an extremely sensitive and highly selective analytical measurement method. It employs lasers (laser) to eject electrons (electron) from selected types of atoms or molecules, splitting the neutral species into a positive ion (cation) and a free electron with a negative charge. Those ions or electrons are then detected and counted by various means to identify elements or compounds and determine their concentration in a sample. The RIS method was originated in the 1970s and is now used in a growing number of applications to advance knowledge in physics, chemistry, and biology. It is applied in a wide variety of practical measurement systems because it offers the combined advantages of high selectivity between different types of atoms and sensitivity at the one-atom level.

      Applications of a simple atom counter include physical and chemical studies of defined populations of atoms. More advanced systems incorporate various forms of mass spectrometers, which offer the additional feature of isotopic selectivity. These more elaborate RIS systems can be used, for instance, to date lunar materials and meteorites, study old groundwater and ice caps, measure the neutrino output of the Sun, determine trace elements in electronic-grade materials, search for resources such as oil, gold, and platinum, study the role of trace elements in medicine and biology, determine DNA structure, and address a number of environmental problems.

Ionization processes
Basic energy considerations
      A basic understanding of atomic structure is necessary for the study of resonance ionization (see above Foundations of atomic spectra: Basic atomic structure (spectroscopy)). Unless an atom is subjected to some external influence, it will be in the state of lowest energy (ground state) in which the electrons systematically fill all the orbits from those nearest the nucleus outward to some larger orbit containing the outermost (valence) electrons. A valence electron can be promoted to an orbit even farther from the nucleus if it absorbs a photon. To initiate the excitation, the photon must have an energy that lies within a very narrow range, as the energies of all the orbits surrounding the nucleus, including the unfilled ones, are rigorously prescribed by quantum mechanics. Each element has its own unique set of energy levels, which is the foundation for both emission spectroscopy and absorption spectroscopy. ionization of an atom occurs when an electron is completely stripped from the atom and ejected into the ionization continuum. The gap between energy possessed by an atom in its ground state and the energy level at the edge of the ionization continuum is the ionization potential (ionization energy).

 The photon energies used in the resonance (stepwise) ionization of an atom (or molecule) are too low to ionize the atom directly from its ground state; thus at least two steps are used. The first absorption is a resonance process as illustrated in the examples in Figure 14—>, and this assures that the ionization will not be observed unless the laser is tuned to the atom—i.e., operating at the appropriate wavelength. Quantum mechanics does not restrict the energy of free electrons in the continuum, and so a photon of any minimum energy can be absorbed to complete the resonance-ionization process.

      With certain pulsed lasers, the two-photon RIS process can be saturated so that one electron is removed from each atom of the selected type. Furthermore, ionization detectors can be used to sense a single electron or positive ion. Therefore, individual atoms can be counted. By taking advantage of tunable laser technology to implement a variety of RIS schemes, it is feasible to detect almost every atom in the periodic table. The combined features of selectivity, sensitivity, and generality make RIS suitable for a wide variety of applications.

RIS schemes
  A simple scheme in which two photons from the same laser cause resonance ionization of an atom is illustrated in Figure 14—>. A single wavelength must be chosen to excite the atom from its ground state to an excited state, while the second photon completes the ionization process. For example, to achieve resonance ionization in the cesium atom that has an ionization potential of only 3.9 electron volts, the scheme of Figure 14A—> works well with a single-colour laser at the wavelength of 459.3 nanometres, or a photon energy of about 2.7 electron volts. (Photon energies and atomic energy levels are given in units of electron volts [eV], or in wavelength units of nanometres [nm]. A useful and approximate relationship between the two is easy to remember since eV = 1,234/nm.) Similar schemes have been used for other alkali atoms because these atoms also have low ionization potentials.

   For most atoms, more elaborate resonance-ionization schemes than the simple two-step process shown in Figure 14A—> are required. The higher the ionization potential of the atom, the more complex is the process. For example, the inert element krypton has an ionization potential of 14.0 electron volts and requires a more elaborate RIS scheme of the type shown in Figure 14B—>. The first step is a resonance transition at the wavelength of 116.5 nanometres, followed by a second resonance step at 558.1 nanometres. Subsequent ionization of this second excited state is accomplished with a long wavelength, such as 1,064 nanometres. Generation of the 116.5-nanometre radiation requires a complex laser scheme. Another useful type of RIS scheme is shown in Figure 14C—>. In this method the atom is excited to a level very near the ionization continuum and exists in a so-called Rydberg state. In such a state the electron has been promoted to an orbit that is so far from the nucleus that it is scarcely bound. Even an electric field of moderate strength can be pulsed to remove the electron and complete the resonance-ionization process. With the schemes discussed above and reasonable variations of them, all the elements in nature can be detected with RIS except for two of the inert gases—helium and neon.

Lasers for RIS
      The essential components of RIS methods are tunable lasers, which can be of either the pulsed or the continuous-wave variety. Pulsed lasers are more frequently used since they can add time resolution to a measurement system. In addition, pulsed lasers produce high peak power, permitting the efficient use of nonlinear optics to generate short-wavelength radiations. For example, in frequency doubling, photons of frequency ω1 incident to a crystal will emerge from the crystal with frequencies ω1 and 2ω1, where the component 2ω1 can have a large fraction of the intensity of ω1. Nonlinear processes are efficient when laser beams are intense, a condition that favours pulsed lasers but that does not exclude the use of certain types of continuous-wave lasers. For each atom, the volume that can be saturated in the RIS process depends on the laser energy per pulse and other aspects of the laser.

      Practical information on a wide variety of useful lasers can be obtained by consulting references listed in the Bibliography (spectroscopy).

atom counting
      The concept of the atom is an ancient one; the Greek philosopher Democritus (c. 460–c. 370 BC) proposed a form of “atomism” (atomism) that contained the essential features of the chemical atom later introduced by the British chemist John Dalton (Dalton, John) in 1810. The British physicist Ernest Rutherford (Rutherford, Ernest, Baron Rutherford of Nelson, of Cambridge) spoke of counting the atoms and in 1908, with the German physicist Hans Geiger (Geiger, Hans), disclosed the first electrical detector for ionizing radiations. The development of wavelength-tunable lasers has made it possible to carry out Rutherford's concept of counting atoms. As stated above, RIS can be used to remove one electron from each of the atoms of a selected type, and the modern version of the electrical detector, known as the proportional counter, can even be made to count a single electron. Thus, all that is required for the most elementary form of atom counting is to pulse the proper laser beam through a proportional counter.

 Experimental demonstrations of atom counting can be performed by introducing low concentrations of cesium vapour into proportional counters, commonly used for nuclear radiation detection, that contain a “counting” gas composed of a mixture of argon and methane. Pulsed laser beams used to implement the RIS scheme of Figure 14A—> can be directed through a proportional counter to detect individual atoms of cesium without interference from the much larger number of argon atoms and methane molecules.

Resonance-ionization mass spectrometry
      For the purpose of determining the relative weights (atomic weight) of atomic nuclei, the mass spectrometer is one of the most useful instruments used by analytical chemists. If two atoms with the same number of protons (proton) (denoted Z) contain different numbers of neutrons (neutron), N, they are referred to as isotopes; (isotope) if they have the same atomic mass, A, (Z + N) but have different numbers of protons, they are called isobars (isobar). Mass spectrometers are well suited to the measurement of isotopes, but they have difficulty in resolving isobars of nearly equal masses. The incorporation of RIS, which is inherently a Z-selective process, solves the isobar problem. Furthermore, RIS, when operated near saturation, provides a considerably more sensitive ionization source for the mass spectrometer than does the conventional electron gun. The combined technique, called resonance-ionization mass spectrometry (RIMS), also eliminates the problems arising from molecular background ionization that occur when using conventional electron guns. In the RIMS method, interferences due to these molecular ions are greatly reduced, again due to the inherent selectivity of the RIS process.

      Since then the quadrupole mass filter and the time-of-flight mass spectrometer have been developed. These three types have been built into RIMS systems (see mass spectrometry).

noble gas detection
 As discussed above, RIS can be applied to the inert, or noble, gases only with great difficulty due to the short wavelength required for the first excitation step. The detection of specific isotopes of the noble gases, such as krypton-81 (81Kr), is quite important. Consequently, the system shown in Figure 15—> was developed to demonstrate that RIS can be used for counting small numbers of krypton-81 atoms. The purpose of this apparatus is essentially to carry out the concept of the sorting demon introduced by the Scottish physicist James Clerk Maxwell (Maxwell, James Clerk), which was of considerable interest to physicists in the late 1800s in connection with the second law of thermodynamics, or the entropy principle. Thus, the experimental objective is to detect all the krypton-81 atoms and count them individually, even when mixed with enormously larger numbers of krypton-82 atoms, other isotopes of krypton, and many other types of atoms or molecules. The scheme involves first achieving Z-selectivity using RIS to sort krypton, followed with A-selectivity using the quadrupole mass filter. It is necessary to include an “atom buncher” to increase the chance that a krypton atom will be in the laser beam when the beam is pulsed through the apparatus. The atom buncher consists of a surface held near the temperature of liquid helium to condense the krypton atoms and another pulsed laser to heat the surface just prior to the application of the RIS laser pulse. Following resonance ionization, the inert atoms are implanted into the detector, which removes them from the vacuum portion of the apparatus where they were initially confined. As each ion is implanted, a small number of electrons are emitted, and these pulses are counted to determine the number of implanted atoms. The process is continued until nearly all the krypton-81 atoms are counted. Variations of the design of this apparatus have included implementing a time-of-flight mass spectrometer for the selection of krypton-81 or another isotope.

      Because of the long radioactive-decay (radioactivity) half-life (210,000 years) of krypton-81, it is impossible to determine small numbers of these atoms by decay counting. Because the RIS method can count the small numbers of krypton-81 atoms, it can be used for dating polar ice to obtain histories of the climate to about one million years ago and also for studying the history of glaciers. Dating of groundwater up to one million years old is an important application for the study of hydrology and for knowledge on the safe deposition of nuclear wastes. Also, analysis of krypton-81, along with at least one of the stable isotopes of krypton, provides a method for obtaining the cosmic-ray exposure ages of lunar materials and meteorites.

Neutrino detection
      Radiochemical experiments, conducted deep beneath the Earth's surface to shield out cosmic rays, have revealed much new information about the Sun and about the properties of neutrinos (neutrino) (electrically neutral, virtually massless particles) emitted from its active core. In large vats filled with solutions rich in chlorine atoms, the flux from the boron-8 (8B) source of solar neutrinos can convert a few of the chlorine-37 (37Cl) atoms to argon-37 (37Ar) atoms with a half-life of 35 days. These atoms can then be detected by nuclear decay counting to determine the flux of the high-energy neutrinos striking the Earth. A similar experiment for detecting the much larger flux of the beryllium-7 (7Be) neutrinos of lower energy can now be done because of the ability to count a small number of krypton-81 atoms produced by neutrino capture in bromine-81 (81Br). Since the atoms are counted directly without waiting for radioactive decay, the 210,000-year half-life of krypton-81 is not an impediment.

RIS atomization methods
Thermal atomization
   Because the RIS technique is limited to the study of free atoms or molecules in the gas phase, the analysis of solids and liquids requires a means for releasing atoms from the bulk material. A simple and effective system in which samples are atomized with a graphite oven is illustrated in Figure 16—>. A small solid or liquid sample is placed into the graphite oven, which is electrically heated to more than 2,000° C. As the sample evaporates, it dissociates into a plume containing free atoms, some of which are ionized with pulsed RIS. In the illustration of Figure 16—>, a RIS scheme similar to that of Figure 14C—> is used, in which the final stage in the ionization process is accomplished by pulsing an electric field onto the atoms in a high Rydberg state. Following ion extraction, mass analysis is performed with a time-of-flight technique to eliminate isobars and unwanted molecular ion fragments.

      Substantial work is accomplished with thermal atomization methods. With detection limits of less than one part per trillion, the graphite furnace version can be installed aboard ships to explore (undersea exploration) the ocean for noble metals such as gold, platinum, and rhodium. In another important application to the Earth sciences, the furnace technique is used to study the rhodium content of geologic samples associated with the great Mesozoic extinction of 65 million years ago. Correlation of the concentrations of rhodium and iridium, the latter determined by neutron-activation analysis, has provided much support to the theory that the high concentration of iridium found in the Cretaceous-Tertiary boundary was caused by a large body of cosmic origin falling on the Earth. Analysis of samples taken from this boundary show that the ratio of iridium to rhodium is about the same as the ratio found in meteorites, and this strengthens the theory that a cosmic body striking the Earth caused mass extinction of the biological species associated with the Mesozoic Era, including the dinosaurs.

      Filamentary heating methods also are utilized for important geologic research. For instance, the age of rocks is determined by measuring the amounts of isotopes of rhenium and osmium. The isotope rhenium-187 (187Re) decays to osmium-187 (187Os) having a half-life of 43 billion years; hence, the Re-Os system can be used to determine when geologic materials were solidified in the Earth.

      Thermal techniques are producing significant practical results in the exploration of natural resources, medical research and treatment, and environmental research. An especially impressive example of exploration is taking place in China, where RIS is used to sample gold, platinum, and other precious metals in water streams to locate ore deposits. Since the average concentration of gold in fresh water is only 0.03 parts per billion, the analytical methods employed must be extremely sensitive and selective against other species in the sample.

Sputter atomization
      When energetic particles (such as 20-keV [thousand electron volts] argon ions) strike the surface of a solid, neutral atoms and secondary charged particles are ejected from the target in a process called sputtering. In the secondary ion mass spectrometry (SIMS) method, these secondary ions are used to gain information about the target material (see mass spectrometry: General principles: Ion sources: Secondary-ion emission (mass spectrometry)). In contrast, the sputter-initiated RIS (SIRIS) method takes advantage of the much more numerous neutral atoms emitted in the sputtering process. In SIRIS devices the secondary ions are rejected because the yield of these ions can be greatly affected by the composition of the host material (known as the matrix effect). Ion sputtering, in contrast to thermal atomization, can be turned on or off in short pulses; for this reason, good temporal overlap with the RIS beams is achievable. This feature allows better utilization of small samples.

      Analysis of high-purity semiconducting materials for the electronics industry is one of the principal applications of the SIRIS method. The method can detect, for example, indium in silicon at the one part per trillion level. The high efficiency of the pulsed sputtering method makes it possible to record one count due to indium at the detector for only four atoms of indium sputtered from the solid silicon target. Analyses of interfaces are of growing importance as electronic circuits become more compact, and in such designs matrix effects are of great concern. Matrix effects are negligible when using the SIRIS method for depth-profiling a gold-coated silicon dioxide–indium phosphide (SiO2/InP) sample.

      RIS methods are applied in the study of basic physical and chemical phenomena in the surface sciences. Knowledge of the interactions of energetic particle beams with surfaces is important in several areas, such as chemical modification of electronics materials, ion etching, ion implantation, and surface chemical kinetics. For these applications, RIS provides the capability to identify and measure the neutral species released from surfaces in response to stimulation with ion probes, laser beams, or other agents.

      Other applications of the SIRIS method are made in medicine, biology, environmental research, geology, and natural resource exploration. Sequencing of the DNA molecule is a significant biological application, which requires that spatial resolution be incorporated into the measurement system. SIRIS is also increasingly becoming utilized in the imaging of neutral atoms.

Additional applications of RIS
On-line accelerator applications
      In the above examples it is not necessary for the RIS process to be isotopically selective. Normal spectroscopic lines, however, are slightly affected by nuclear properties. There are two effects: the general shift due to the mass of the nucleus, known as the isotope shift, and a more specific effect depending on the magnetic properties of nuclei known as hyperfine structure. These optical shifts are small and require high resolution in the wavelengths of the lasers. RIS methods coupled with isotopic selectivity can be extremely useful in nuclear physics.

      Rare species that are produced by atomic or nuclear processes in accelerator experiments are extensively studied with RIS. An isotope accelerator delivers ions of a particular isotope into a small oven where the short-lived nuclei decay. After a brief accumulation time, the furnace creates an atomic beam containing the decay products. These decay products are then subjected to the RIS process followed by time-of-flight analysis of the ions. Analysis of the optical shifts leads to information on magnetic moments of nuclei and on the mean square radii of the nuclear charges. Such measurements have been performed on several hundred rare species, and these studies continue at various laboratories principally in Europe, the United States, and Japan.

Molecular applications
      While most applications of RIS have been made with free atoms, molecular studies are increasingly important. With simple diatomic molecules such as carbon monoxide (CO) or nitric oxide (NO), the RIS schemes are not fundamentally different from their atomic counterparts, except that molecular spectroscopy is more complex and must be understood in detail for routine RIS applications. On the other hand, RIS itself is a powerful tool for the study of molecular spectroscopy, even for the study of complex organic molecules of biological importance.

George Samuel Hurst

Additional Reading
J. Michael Hollas, Modern Spectroscopy (1987), is a broad introductory-level presentation. Optical data, X-ray data, samples of optical spectra of some materials, tabulation of wavelengths, and details of methods of radiation detection may be found in Dwight E. Gray (ed.), American Institute of Physics Handbook, 2nd ed. (1963). J.W. Robinson, Practical Handbook of Spectroscopy (1991), lists a range of spectroscopic data covering X-ray and neutron spectroscopy, photoelectron spectroscopy, ultraviolet, optical, and infrared spectroscopy.Wolfgang Demtröder, Laser Spectroscopy (1981); and Stig Stenholm, Foundations of Laser Spectroscopy (1984), discuss many of the basic concepts and instrumentation of laser spectroscopy. Y.R. Shen, The Principles of Nonlinear Optics (1984), focuses on nonlinear spectroscopic techniques made available with lasers. Murray Sargent III, Marlan O. Scully, and Willis E. Lamb, Jr., Laser Physics (1974), is a reference text on the theory of the laser. Anthony E. Siegman, Lasers (1986), provides an updated and extensive discussion of laser physics, optical beams and resonators, Q-switching, and mode-locking.Principles of atomic spectroscopy are discussed in Hans A. Bethe and Edwin E. Salpeter, Quantum Mechanics of One- and Two-electron Atoms (1957, reissued 1977), an authoritative account of the basic quantum mechanics of hydrogen- and helium-like atoms; and Igor I. Sobelman, Atomic Spectra and Radiative Transitions, 2nd ed. (1992), a more modern version. Alan Corney, Atomic and Laser Spectroscopy (1977), covers the foundations of atomic physics and the interactions of electromagnetic radiation with atoms plus applications at the advanced undergraduate level. Extensive tabulations of wavelengths of the elements are in Charlotte E. Moore, Atomic Energy Levels as Derived from the Analysis of Optical Spectra, 3 vols. (1949–58, reprinted 1971). Tabulations of the lifetimes of excited states of the elements are listed in W.L. Wiese, M.W. Smith, and B.M. Glennon, Atomic Transition Probabilities (1966– ).Introductions to molecular spectroscopy are provided in the books by Demtröder and by Stenholm, cited earlier; and by Jack D. Graybeal, Molecular Spectroscopy (1988), which concentrates on the development of fundamental relationships; Marlin D. Harmony, Introduction to Molecular Energies and Spectra (1972), an intermediate-level introduction to the primary areas of spectroscopy; Jeffrey I. Steinfeld, Molecules and Radiation: An Introduction to Modern Molecular Spectroscopy, 2nd ed. (1985), an intermediate-level introduction to general principles and selected areas; E. Bright Wilson, Jr., J.C. Decius, and Paul C. Cross, Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra (1955, reprinted 1980), the definitive treatment of the fundamentals; and Gerhard Herzberg, Molecular Spectra and Molecular Structure, 4 vol. (1939–79), with a 2nd ed. of vol. 1 (1950), comprising the most comprehensive and advanced-level treatment of basic concepts. Advanced-level treatments include Harry C. Allen, Jr., and Paul C. Cross, Molecular Vib-rotors (1963), on rotation and vibration; Walter Gordy and Robert L. Cook, Microwave Molecular Spectra, 3rd ed. (1984), a comprehensive treatment; and J. Michael Hollas, High Resolution Spectroscopy (1982), a review of all areas with a minimum of mathematical development.Studies of X-ray and radio-frequency spectroscopy include Arne Eld Sandström, “Experimental Methods of X-ray Spectroscopy: Ordinary Wavelengths,” in Handbuch der Physik, vol. 30 (1957), pp. 78–245, a survey of X-ray spectroscopy; and B.E. Warren, X-ray Diffraction (1969, reprinted 1990), an authoritative treatment. Herman Winick and S. Doniach (eds.), Synchrotron Radiation Research (1980), covers many X-ray spectroscopy techniques made possible with synchrotron sources.Steven Chu Jack D. Graybeal Two textbooks deal exclusively with resonance-ionization spectroscopy: Vladilen S. Letokhov, Laser Photoionization Spectroscopy (1987), reviews the general subject with considerable detail on laser schemes and applications, including an excellent account of the early work in the Academy of Sciences of the U.S.S.R.; and G.S. Hurst and M.G. Payne, Principles and Applications of Resonance Ionisation Spectroscopy (1988), covers the early experiments and relevant theory on resonance ionization. International symposia on RIS have convened on approximately a two-year cycle since 1981, and the more recent proceedings from them are published with the title Resonance Ionization Spectroscopy, e.g., Resonance Ionization Spectroscopy 1990, ed. by J.E. Parks and N. Omenetto (1991).George Samuel Hurst

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