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Biography
redirect|Phosphorescent|the indie rock band|Phosphorescent (band)dablink|In common use, phosphorescence also refers to the emission of light by bioluminescent plankton, and some other forms of chemoluminescence .Commonly seen examples of phosphorescent materials are the glow-in-the-dark toys, paint, and clock dials that glow for some time after being charged with a bright light such as in any normal reading or room light. Typically the glowing then slowly fades out within minutes (or up to a few hours) in a dark room.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
The study of phosphorescent materials led to the Radioactivity#Discovery|discovery of radioactivity in 1896.
Explanations
Simple
Quantum mechanical
Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, on the order of 10 nanosecond s. Light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity ( see term symbol ), usually a triplet state . As a result, the energy can become trapped in the triplet state with only classically "forbidden" transitions available to return to the lower energy state. These transitions, although "forbidden", will still occur in quantum mechanics but are chemical kinetics|kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called "glow-in-the-dark" materials.
Equation
: where S is a singlet state|singlet and T a spin triplet|triplet whose subscripts denote states (0 is the ground state, and 1 the excited state). Transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity.
Chemiluminescence
main|ChemiluminescenceSome examples of "glow-in-the-dark" materials do not glow by phosphorescence. For example, " glow sticks " glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemiluminescence, an excited state is created via a chemical reaction. The light emission tracks the kinetic progress of the underlying chemical reaction. The excited state will then transfer to a "dye" molecule, also known as a sensitizer or fluorophor , and subsequently fluoresce back to the ground state
Materials
Common pigments used in phosphorescent materials include zinc sulfide and strontium aluminate . Use of zinc sulfide for safety related products dates back to the 1930s. However, the development of strontium oxide aluminate, with a luminance approximately 10 times greater than zinc sulfide, has relegated most zinc sulfide based products to the novelty category. Strontium oxide aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.Zitoun, D.; Bernaud, L.; Manteghetti, A. Microwave Synthesis of a Long-Lasting Phosphor. J. Chem. Ed. 2009, 86, 72-75.DOI|10.1021/ed086p72
