Phosphorescence

Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter threshold wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed. There is no distinct boundary between the emission times of fluorescence and phosphorescence (i.e. if a substance glows under a black light it is generally considered fluorescent, and if it glows in the dark it is often simply called phosphorescent). The phenomena can usually be classified by the three different mechanisms that produce the light and the typical timescales at which they emit light: fluorescence, triplet phosphorescence, and persistent phosphorescence. Fluorescent materials stop emitting light within nanoseconds (billionths of a second) after the excitation radiation is removed, while phosphorescent materials may continue to emit an afterglow ranging from a few microseconds to many hours after the excitation is removed. There are two separate mechanisms that may produce phosphorescence, called triplet phosphorescence (or simply phosphorescence) and persistent phosphorescence (or persistent luminescence): Triplet phosphorescence occurs when an atom absorbs a high-energy photon, and the energy becomes locked in the spin multiplicity of the electrons, generally changing from a fluorescent singlet state to a slower emitting triplet state. The slower timescales of the reemission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur relatively slowly in certain materials, absorbed radiation is reemitted at a lower intensity, ranging from a few microseconds to as much as one second after the excitation is removed. Persistent phosphorescence occurs when an atom absorbs a high-energy photon and an electron becomes trapped...