luminescence
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luminescence
Luminescence
Light emission that cannot be attributed merely to the temperature of the emitting body. Various types of luminescence are often distinguished according to the source of the energy which excites the emission. When the light energy emitted results from a chemical reaction, such as in the slow oxidation of phosphorus at ordinary temperatures, the emission is called chemiluminescence. When the luminescent chemical reaction occurs in a living system, such as in the glow of the firefly, the emission is called bioluminescence. In the foregoing two examples part of the energy of a chemical reaction is converted into light. There are also types of luminescence that are initiated by the flow of some form of energy into the body from the outside. According to the source of the exciting energy, these luminescences are designated as cathodoluminescence if the energy comes from electron bombardment; radioluminescence or roentgenoluminescence if the energy comes from x-rays or from γ-rays; photoluminescence if the energy comes from ultraviolet, visible, or infrared radiation; and electroluminescence if the energy comes from the application of an electric field. By attaching a suitable prefix to the word luminescence, similar designations may be coined to characterize luminescence excited by other agents. Since a given substance can frequently be made to luminesce by a number of different external exciting agents, and since the atomic and electronic phenomena that cause luminescence are basically the same regardless of the mode of excitation, the classification of luminescence phenomena into the foregoing categories is only a matter of convenience, not of fundamental distinction.
When a luminescent system provided with a special configuration is excited, or “pumped,” with sufficient intensity of excitation to cause an excess of excited atoms over unexcited atoms (a so-called population inversion), it can produce laser action. (Laser is an acronym for light amplification by stimulated emission of radiation.) This laser emission is a coherent stimulated luminescence, in contrast to the incoherent spontaneous emission from most luminescent systems as they are ordinarily excited and used. See Laser, Optical pumping
A second basis frequently used for characterizing luminescence is its persistence after the source of exciting energy is removed. Many substances continue to luminesce for extended periods after the exciting energy is shut off. The delayed light emission (afterglow) is generally called phosphorescence; the light emitted during the period of excitation is generally called fluorescence. In an exact sense, this classification, based on persistence of the afterglow, is not meaningful because it depends on the properties of the detector used to observe the luminescence. With appropriate instruments one can detect afterglows lasting on the order of a few thousandths of a microsecond, which would be imperceptible to the human eye. The characterization of such a luminescence, based on its persistence, as either fluorescence or phosphorescence would therefore depend upon whether the observation was made by eye or by instrumental means. These terms are nevertheless commonly used in the approximate sense defined here, and are convenient for many practical purposes. However, they can be given a more precise meaning. For example, fluorescence may be defined as a luminescence emission having an afterglow duration which is temperature-independent, while phosphorescence may be defined as a luminescence with an afterglow duration which becomes shorter with increasing temperature. See Cathodoluminescence, Electroluminescence, Fluorescence, Phosphorescence, Photoluminescence, Thermoluminescence
luminescence
[‚lü·mə′nes·əns]luminescence
Luminescence
radiation that is in excess of the thermal radiation of a body and continues for a time interval significantly exceeding the period of light oscillations.
The first part of the definition differentiates luminescence from thermal equilibrium radiation and shows that the concept of luminescence is applicable only to the aggregate of atoms or molecules that are in a state close to equilibrium, since the idea of thermal radiation or luminescence is meaningless for states that are grossly out of equilibrium. Thermal radiation in the visible spectral region is noticeable only at temperatures of several hundred or thousand degrees, whereas the same object may luminesce at any temperature. For this reason luminescence is frequently called a cold glow. The second part of the definition (the property of duration) was introduced by S. I. Vavilov to distinguish luminescence from various kinds of scattering and reflection of light, parametric conversion of light, bremsstrah-lung, and Cherenkov radiation. Luminescence differs from various kinds of scattering in that in luminescence intermediate processes, whose duration is greater than the period of a light wave, occur between absorption and emission. As a result, the correlation between the oscillation phases of absorbed and radiated light is lost.
The concept of luminescence originally included only radiation of visible light; currently, it is also applied to radiation in the near-ultraviolet and infrared regions.
Natural luminescence phenomena, such as the aurora borealis and the glowing of some insects and minerals and of rotting wood, have been known since early antiquity. However, systematic study of luminescence was not undertaken until the late 19th century (E. and A. Becquerel, P. Lenard, and W. Crookes). W. K. Roentgen’s interest in studying the glow of various substances led to his discovery of X rays; in 1896, while studying phosphors, A. Becquerel discovered the phenomenon of radioactivity. An exceptionally important contribution to the determination of the basic laws of luminescence was made by the Soviet school of physicists established by S. I. Vavilov.
Luminescence may be classified according to the type of excitation, the mechanism of energy conversion, or the time characteristics of the luminosity. According to the type of excitation, a distinction is made among photoluminescence (excitation by light); radioluminescence (excitation by penetrating radiation), special cases of which are X-ray luminescence (excitation by X rays), cathode luminescence (excitation by an electron beam), ion luminescence (excitation by accelerated ions), and alpha luminescence (excitation by alpha particles); electroluminescence (excitation by an electric field); triboluminescence (excitation by mechanical stresses); chemiluminescence (excitation as a result of chemical reactions); and candoluminescence (excitation caused by recombination of radicals on the surface).
According to the duration of the luminosity, a distinction is made between fluorescence (a rapidly decaying luminescence) and phosphorescence (prolonged luminescence). This distinction is arbitrary, since it is not possible to determine a rigidly defined time limit; such a limit depends on the time resolution of the recording apparatus.
In terms of the mechanism of the elementary processes, a distinction is made among resonance, spontaneous, forced, and recombination luminescence. The elementary event of luminescence (see Figure 1) consists in the absorption of energy, with transition of the atom or molecule from the ground state (1) to the excited state (3); a nonradiative transition to state (2); and a radiative transition to the ground state (1). In a special case the radiation of luminescence can occur during the transition of the atom or molecule from state (3) to state (1). In this case the luminescence is called resonance luminescence. Resonance luminescence is most frequently observed in atomic pairs (mercury, cadmium, sodium, and so on), in several simple molecules, and in impurity crystals.
In most cases the probability of transition of an atom or molecule from state (3) to state (2) is greater than the probability of a direct transition to the ground state (1). State (2) most
frequently lies below the absorption state (3); therefore, part of the energy is lost as heat (that is, vibration of atoms is excited) and a quantum of luminescent light has less energy (and a longer wavelength) than the quanta of exciting light (Stokes’ law). However, anti-Stokes luminescence may also be observed. In this case, a molecule makes the transition to radiation state (2), which is higher than state (3), because of absorption of oscillatory energy. In anti-Stokes luminescence the energy of the emitted quantum is greater than the energy of the exciting quantum, and the luminescence intensity is low.
Radiation state (2) may belong to the same atom or molecule that absorbed the excitation energy or to other atoms (in the first instance the atom is called the center of luminescence and the transition is called intracentral). In the simplest case, when the excitation energy remains within the same atom, the luminescence is called spontaneous. This kind of luminescence is characteristic of atoms or molecules in vapors and solutions and of impurity atoms in crystals. In some cases (see Figure 2) the atom or molecule occupies the intermediate, metastable state (4) before moving to the radiation state (2). For a transition to the radiation state, additional energy—for example, the energy of thermal motion or infrared light—must be imparted to the atom. The luminescence arising in such processes is called metastable (stimulated).
Luminescence processes in various substances differ mainly in the mechanism of particle transition from the absorption state (3) to the radiation state (2). Energy transfer to other atoms or molecules is accomplished by electrons during electronion impact, in processes of ionization and recombination, or by exchange during a direct collision of an excited atom with a nonexcited atom. Because of the low concentration of atoms in gases, resonance and exchange processes of energy transfer play a minor role. They become substantial in condensed mediums. In such processes the excitation energy may also be transferred by oscillation of nuclei. Finally, in crystals, the transfer of energy through conduction electrons, holes, and electron-hole pairs (excitons) becomes the determining factor. If the concluding event of energy transfer is recombination (restoration of particles—for example, electrons and ions or electrons and holes), the luminescence accompanying the process is called recombinational.
The ability to luminesce is exhibited by various substances. To be able to luminesce, a substance must have a discrete spectrum—that is, its levels must be separated by forbidden energy bands. Therefore, metals in the solid or liquid phase, which have a continuous energy spectrum, do not exhibit luminescence; in metals the excitation energy is continuously converted to heat.
A second necessary condition for luminescence demands that the probability of radiative transitions be greater than that of nonradiative transitions. An increase in the probability of nonradiative transitions results in quenching of luminescence. The probability of nonradiative transitions depends on many factors; it increases with increased temperature (temperature quenching) and concentration of luminescing molecules (concentration quenching) or impurities (impurity quenching). Such quenching of luminescence is associated with the transfer of excitation energy to the quenching agent or loss of the energy during interaction of luminescing molecules among themselves and with the thermal vibrations of the medium. Consequently, the ability to luminesce depends on the nature of the luminescing substance and its phase state, as well as on external conditions. At low pressures, metal vapors and noble gases will luminesce (this phenomenon is used in gas-discharge light sources, fluorescent lamps, and gas lasers). Luminescence of liquid mediums is mainly characteristic of solutions of organic substances.
The brightness of luminescent crystals depends on their impurity (activator) content. The energy levels of such activators may be absorption, intermediate, or radiation states. The role of the states may also be played by valence and conduction energy bands. Crystals exhibiting luminescence are called crystal phosphors.
In crystal phosphors, excitation by light, electric current, or a beam of particles generates free electrons, holes, and excitons (Figure 3). Electrons may migrate along the lattice, settling in the traps. Luminescence occurring during recombination of free electrons and holes is called interband luminescence (a). If an electron recombines with a hole that has been captured by a center of luminosity (an impurity atom or lattice defect), center luminescence occurs. Recombination of excitons produces exciton luminescence (c). The spectrum of crystal phosphors consists of interband, exciton, and impurity bands.
The main physical characteristics of luminescence are the method of excitation (for photoluminescence, the excitation spectrum), the radiation spectrum (the study of luminescent radiation spectra is a branch of spectroscopy), the state of polarization of the radiation, and radiation efficiency, or the ratio of radiated energy to absorbed energy (for photoluminescence, the concept of quantum efficiency of luminescence is introduced; it is the ratio of radiated quanta to absorbed quanta). Polarization of luminescence is associated with orientation and multipolarity of the radiation and absorbing atom systems.
An important characteristic of luminescence is its kinetics (the dependence of luminosity on time, the radiation intensity I, and excitation intensity, as well as other factors, such as temperature). The kinetics of luminescence depends strongly on elementary processes. The kinetics of decay of resonance luminescence at low excitation densities and low concentrations of excited atoms is exponential: I = I0e1/T where τ characterizes the lifetime at the excitation state and is equal to the reciprocal of the probability of a spontaneous transition per unit time, and t is the duration of the luminosity. A deviation from the exponential quenching law caused by processes of induced radiation is observed at high excitation densities. The quantum efficiency of resonance luminescence usually approaches 1. The kinetics of quenching of spontaneous luminescence is also usually exponential. The kinetics of recombination luminescence is complex and is determined by the probabilities of recombination, capture, and release of electrons by traps (the probabilities are dependent on temperature). A hyperbolic decay law is most frequently encountered: where p is a constant and a usually has a value between 1 and 2. The decay time of luminescence varies within wide limits (from 108 sec to several hours). When quenching processes occur, the luminescence efficiency and decay time are decreased. Study of the kinetics of quenching of luminescence yields important information on processes of molecular interaction and on energy migration.
The study of the spectrum, kinetics, and polarization of radiation of luminescence makes possible the study of the spectrum of energy states of a substance, the spatial structure of molecules, and the processes of energy migration. Instruments used in the study of luminescence, which record the luminosity and its distribution within the spectrum, are called spectrophotometers. Tau meters and fluorometers are used to measure decay times. Luminescence methods are of utmost importance in solid-state physics. The operation of lasers is based on the luminescence of some substances. The luminescence of a number of living things has made it possible to obtain information about processes that take place in cells at the molecular level. In research on crystal phosphors, the conduct of parallel studies of their luminescence and conductivity is very fruitful.
The broad study of luminescence also results from the importance of its practical applications. The brightness and high energy output of luminescence made possible the design of high-efficiency fluorescent light sources based on electroluminescence and photoluminescence. The bright luminescence of a number of substances brought about the development of a method for detecting small quantities of impurities, for sorting substances according to their luminescent properties, and for studying mixtures, such as petroleum. Cathode luminescence is the basic phenomenon used in luminescent screens for electronic devices (oscilloscopes, television receivers, and radar sets). X-ray luminescence is used in fluoroscopy. The use of radioluminescence was found to be of prime importance in nuclear physics. Luminescence is widely used in film-making and flaw detection. Luminescent dyes are used for fabrics and road signs.
REFERENCES
Pringsheim, P. Fluorestsentsiia i fosforestsentsiia. Moscow, 1951. (Translated from English.)Vavilov, S. I. Sobr. soch., vol. 2. Moscow, 1952. Pages 20, 28, and 29.
Levshin, V. L. Fotoliuminestsentsiia zhidkikh i tverdykh veshchestv. Moscow-Leningrad, 1951.
Antonov-Romanovskii, V. V. Kinetika fotoliuminestsentsii kristallofosforov. Moscow, 1966.
Adirovich, E. I. Nekotorye voprosy teorii liuminestsentsii kristallov. Moscow-Leningrad, 1951.
Fok, M. V. Vvedenie v kinetiku liumenestsentsii kristallofosforov. Moscow, 1964.
Curie, D. Liuminestsentsiia kristallov. Moscow, 1961. (Translated from French.)
Bube, R. Fotoprovodimost’tverdykh tel. Moscow, 1962. (Translated from English.)
E. A. SVIRIDENKOV