LUMINESCENCE

a process by which some materials emit light when they are relatively cool. Familiar examples of luminescence are the light emissions from: electronically excited gases in neon lamps and lightning; tiny inorganic crystals used as coatings in luminescent watch dials, television and radar kinescopes, fluorescent lamps, and X-ray fluoroscope screens; and certain organic materials undergoing oxidation in fireflies and glowworms. Because they luminesce at room temperature such materials emit what is sometimes loosely called cold light, to distinguish it from the temperature-dependent light emitted by incandescent sources. The process of luminescence is started by exciting some material, usually with ultraviolet radiation, X-rays, electrons, alpha particles, electric fields, or energy liberated during some chemical reactions. Suitable materials convert one or more of these invisible input energies to light. Few luminescent materials are efficient enough for practical use. The efficient ones are custom-made to convert a particular input energy to light of a particular colour and intensity. The colour is determined by the material, while the intensity depends on the material and the input energy. Every kind of atom, when alone, will luminesce; and each kind of atom exhibits characteristic spectral lines, which have been interpreted in terms of quantum theory. According to quantum theory an isolated atom or ion can exist indefinitely in an unexcited state (called the ground state) or it can be excited and exist for short periods in one or another of various discrete excited states. In other words, a given kind of atom can exist briefly in one of several separate and distinct states of higher energy, but not in intermediate states. Each state has an energy level that corresponds to a different configuration of the electrons in the atom. When the excited atom drops from a higher to a lower energy level, the difference in energy between the two sharply defined levels is radiated as a discrete bit (quantum) of light that is called a photon. A lone excited atom loses its excess energy by radiation, in the absence of collisions with other atoms. An excited atom in a molecule, however, can dissipate excess energy by converting it to increased agitation of all the atoms that are bound together in the molecule. Spectroscopic analysis of light from luminescing molecules shows that the energy levels of the constituent atoms are altered and proliferated into many additional closely spaced levels as a result of vibrations and rotations of the atomic ensemble. A multiatom ensemble generally has lower efficiency of luminescence than an isolated atom because the assemblage can convert excitation energy into atomic motion, which in condensed matter is thermal agitation (heat). The probability of dissipating input energy as heat is enormously increased on going from an isolated atom to one bound to myriads of others in an elemental liquid or solid. Most elemental liquids and solids, therefore, are nonluminescent. Mercury is an efficient luminescent gas but is a nonluminescent liquid. There are some nonelemental liquids, however, that have relatively high luminescence efficiencies. In benzene C6H6 (which luminesces as a gas, a liquid, and a solid) the hexagonal benzene molecule emits ultraviolet radiation in all three physical states. Crystals are generally the most efficient sources of luminescence, because their ordered structures provide stable arrangements of atoms and permit relatively efficient ingress and internal transport of input energy, and emission of photons. The terms phosphorescence and fluorescence are often used instead of luminescence. While the distinction does not hold absolutely, the two terms are commonly used to designate luminescence that persists after the activating radiation has ceased (phosphorescence) versus luminescence that ceases within about 10-8 seconds after the activating radiation does so (fluorescence). Instances of luminescence may also be distinguished by the manner in which it is produced. Thus, chemiluminescence proceeds from chemical reactions, as the oxidation of luminol by hydrogen peroxide; bioluminescence (q.v.), a subcategory of chemiluminescence, occurs in living creatures such as fireflies or glowworms; triboluminescence occurs when crystals of certain substances, such as sugar, are crushed. Similarly, thermoluminescence, photoluminescence, electroluminescence, and radioluminescence are stimulated by heat, light, electric discharge, and radiation, respectively. emission of light by certain materials when they are relatively cool. It is in contrast to light emitted from incandescent bodies, such as burning wood or coal, molten iron, and wire heated by an electric current. Luminescence may be seen in neon and fluorescent lamps; television, radar, and X-ray fluoroscope screens; organic substances such as luminol or the luciferins in fireflies and glowworms; certain pigments used in outdoor advertising; and also natural electrical phenomena such as lightning and the aurora borealis. In all these phenomena, light emission does not result from the material being above room temperature, and so luminescence is often called cold light. The practical value of luminescent materials lies in their capacity to transform invisible forms of energy into visible light. Additional reading E. Newton Harvey, A History of Luminescence from the Earliest Times Until 1900 (1957), a classical work, deals rather extensively with the historical development of the different types of luminescence, especially bioluminescence and chemiluminescence. E.J. Bowen (ed.), Luminescence in Chemistry (1968), is a textbook for students and researchers. C.A. Parker, Photoluminescence of Solutions (1968), explains in detail the basic principles of luminescence as applied to photoluminescence in solutions, kinetics, apparatus, and analytic applications. M. Zander, Phosphorimetry (1968), is the first modern monograph dealing exclusively with the phosphorescence of organic materials, with a complete bibliography. George G. Guilbault (ed.), Fluorescence: Theory, Instrumentation, and Practice (1967), is a fairly technical account written by outstanding specialists in their respective fields (good background knowledge is necessary). David M. Hercules (ed.), Fluorescence and Phosphorescence Analysis (1966), comprises chapters of different grades of detail covering the luminescence field. G.F.J. Garlick, Luminescence, Handbuch der Physik, vol. 26, pp. 1128 (1958), an extended text (written in English), systematically explains the physical phenomena and theory of luminescence. Marvin C. Goldberg (ed.), Luminescence Applications in Biological, Chemical, Environmental, and Hydrological Sciences (1989), is a complete compilation of original research in a wide variety of areas. Karl-Dietrich Gundermann The Editors of the Encyclopdia Britannica