Editing Phosphorescence (section)

{{Short description|Process in which energy absorbed by a substance is released relatively slowly in the form of light}}
{{redirect|Phosphorescent|the musical act|Phosphorescent (musician)}}
{{Distinguish|Bioluminescence|Chemiluminescence}}
[[File:Phosphorescence.jpg|thumb|Phosphorescent bird figure]]
[[File:Phosphorescent.jpg|thumb|Phosphorescent, [[europium]]-doped, [[strontium]] silicate-aluminate oxide powder under visible light, fluorescing/phosphorescing under long-wave [[UV light]], and persistently phosphorescing in total darkness]]

'''Phosphorescence''' is a type of [[photoluminescence]] related to [[fluorescence]]. When exposed to light (radiation) of a shorter 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.

In a general sense, 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).<ref>''Illuminating Engineering'' -- Illuminating Engineering Society 1954 Page 228</ref> In a modern, scientific sense, the phenomena can usually be classified by the three different mechanisms that produce the light,{{explain|reason=What are those three mechanisms?|date=May 2024}} and the typical timescales during which those mechanisms emit light. Whereas fluorescent materials stop emitting light within nanoseconds (billionths of a second) after the excitation radiation is removed, phosphorescent materials may continue to emit an afterglow ranging from a few microseconds to many hours after the excitation is removed.<ref>''Persistent Phosphors: From Fundamentals to Applications'' by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25</ref>

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 mechanism|forbidden]]" [[energy level|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.<ref>''Persistent Phosphors: From Fundamentals to Applications'' by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25</ref>

* '''Persistent phosphorescence''' occurs when a high-energy photon is absorbed by an atom and its electron becomes trapped in a defect in the [[Crystal lattice|lattice]] of the [[crystal]]line or [[Amorphous materials|amorphous]] material. A defect such as a missing atom ([[vacancy defect]]) can trap an electron like a [[Pitfall trap|pitfall]], storing that electron's energy until released by a random spike of thermal (vibrational) energy. Such a substance will then emit light of gradually decreasing intensity, ranging from a few seconds to up to several hours after the original excitation.<ref>''Persistent Phosphors: From Fundamentals to Applications'' by Jianrong Qiu, Yang Li, Yongchao Jia -- Elsevier 2020 Page 1--25</ref>

Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint, and clock dials that glow after being charged with a bright light such as in any normal reading or room light.  Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.{{explain|reason=This seems to imply that being in a dark room causes the glow to last longer. Is that the case, and if so, why not the opposite (which seems more intuitive)?|date=May 2024}}<ref>Karl A. Franz, Wolfgang G. Kehr, Alfred Siggel, Jürgen Wieczoreck, and Waldemar Adam "Luminescent Materials" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim.  {{doi|10.1002/14356007.a15_519}}</ref>

The study of phosphorescent materials led to the discovery of [[Radioactive decay#History of discovery|radioactive decay]].