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general term applied to all forms of cool light, i.e., light emitted by sources other than a hot, incandescent body, such as a blackbody radiator. Luminescence is caused by the movement of electrons within a substance from more energetic states to less energetic
..... Click the link for more information. produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescencefluorescence
, luminescence in which light of a visible color is emitted from a substance under stimulation or excitation by light or other forms of electromagnetic radiation or by certain other means.
..... Click the link for more information. in that it continues even after the radiation causing it has ceased. Phosphorescence was first observed in the 17th cent. but was not studied scientifically until the 19th cent. According to the theory first advanced by Philipp Lenard, energy is absorbed by a phosphorescent substance, causing some of the electrons of the crystal to be displaced. These electrons become trapped in potential troughs from which they are eventually freed by temperature-related energy fluctuations within the crystal. As they fall back to their original energy levels, they release their excess energy in the form of light. Impurities in the crystal can play an important role, some serving as activators or coactivators, others as sensitizers, and still others as inhibitors, of phosphorescence. Organo-phosphors are organic dyes that fluoresce in liquid solution and phosphoresce in solid solution or when adsorbed on gels. Their phosphorescence, however, is not temperature-related, as ordinary phosphorescence is, and some consider it instead to be a type of fluorescence that dies slowy.
A delayed luminescence, that is, a luminescence that persists after removal of the exciting source. It is sometimes called afterglow.
This original definition is rather imprecise, because the properties of the detector used will determine whether or not there is an observable persistence. There is no generally accepted rigorous definition or uniform usage of the term phosphorescence. In the literature of inorganic luminescent systems, some authors define phosphorescence as delayed luminescence whose persistence time decreases with increasing temperature. According to this usage, luminescence whose persistence time is independent of temperature is called fluorescence regardless of the length of the afterglow; a temperature-independent afterglow of long duration is called simply a slow fluorescence, which implies that the atomic or molecular transition involved is forbidden to a greater or lesser degree by the spectroscopic selection rules. The most common mechanism of phosphorescence in photoconductive inorganic systems, however, occurs when electrons or holes, set free by the excitation process and trapped at lattice defects, are expelled from their traps by the thermal energy in the system and recombine with oppositely charged carriers with the emission of light. See Hole states in solids, Selection rules (physics)
In the organic literature the term phosphorescence is reserved for the forbidden luminescent transition from a metastable energy state M to the ground state G, while the afterglow corresponding to the M→ E→G process (where E is a higher energy state) is called delayed fluorescence. See Fluorescence, Light, Luminescence
luminescence that persists for an appreciable time after the cessation of excitation, in contrast to fluorescence. The division of luminescence into phosphorescence and fluorescence is somewhat arbitrary and essentially obsolete, since it does not reflect the energy conversion mechanism. Phosphorescence sometimes persists for several hours or days and sometimes for only a few microseconds.
The phosphorescence of crystal phosphors occurs when electrons and holes, which are separated during excitation, recombine. In this case, the persistence of the afterglow is associated with the capture of electrons and holes by traps, from which they can be expelled only after acquiring an additional amount of energy that is determined by the depth of the traps. The phosphorescence of complex organic molecules is associated with the molecules’ being in a metastable state; a transition from such a state to the ground state is highly improbable.
The intensity of the phosphorescence of organic molecules decreases with time, usually in accordance with an exponential decay law. The decay law for the phosphorescence of crystal phosphors is complex; however, in some cases, it is approximately described by Becquerel’s equation B = B0 (1 + at)–α, where t is time, a and α are constants, and B0 is the initial intensity. The law is complex because crystal phosphors contain various types of traps. As a rule, an increase in the temperature of a crystal phosphor accelerates the decay of its phosphorescence.
The decay of phosphorescence depends on the intensity of the excitation only in the case of recombination luminescence. For example, the initial stages of the phosphorescence of crystal phosphors are greatly accelerated when the intensity of the excitation is increased. In the late stages, the intensity of the phosphorescence is only slightly dependent on the intensity of the excitation; that is, the decay curves are asymptotic. Exposure to infrared radiation and the application of an electric field affect the phosphorescence of crystal phosphors.