phosphorescence

phosphorescence (fŏs'fərĕs`əns), luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence is distinguished from fluorescence 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.

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phosphorescence

Emission of light from a substance exposed to radiation and persisting as an afterglow after the exciting radiation has been removed. Unlike fluorescence, in which the absorbed light is emitted about 10⁻⁸ second after excitation, in phosphorescence the extra energy absorbed is stored in metastable states and reemitted later. Phosphorescence may last from about 10⁻³ second to days or even years. The term phosphorescence is often applied to luminescence of living organisms, as well.

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phosphorescence

1. Physics

a. a fluorescence that persists after the bombarding radiation producing it has stopped

b. a fluorescence for which the average lifetime of the excited atoms is greater than 10^--8 seconds

2. the light emitted in phosphorescence

3. the emission of light during a chemical reaction, such as bioluminescence, in which insufficient heat is evolved to cause fluorescence

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phosphorescence [‚fäs·fə′res·əns]

(atomic physics)

Luminescence that persists after removal of the exciting source. Also known as afterglow.

Luminescence whose decay, upon removal of the exciting source, is temperature-dependent.

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Phosphorescence

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