Editing Phosphorescence (section)

==Triplet phosphorescence==
[[File:Electronic Processes Involving Light.png|thumb|upright=1.5|After an electron absorbs a photon of high energy, it may undergo vibrational relaxations and intersystem crossing to another spin state.  Again the system relaxes vibrationally in the new spin state and eventually emits light by phosphorescence.]]

Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a [[photon]] of light, are fast, in 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 electron which absorbed the photon (energy) undergoes an unusual [[intersystem crossing]] into an energy state of different (usually higher) ''spin multiplicity'' (''see [[term symbol]]''), usually a [[triplet state]]. As a result, the excited electron can become trapped in the triplet state with only [[forbidden mechanism|"forbidden" transitions]] available to return to the lower energy singlet 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 decay-times in the order of milliseconds.

Common examples include the phosphor coatings used in [[fluorescent lamp]]s, where phosphorescence on the order of milliseconds or longer is useful for filling in the "off-time" between [[AC current]] cycles, helping to reduce "flicker". Phosphors with faster decay times are used in applications like the pixels excited by free electrons ([[cathodoluminescence]]) in [[cathode-ray tube]] [[Television set|television-set]]s, which are slow enough to allow the formation of a picture as the electron beam scans the screen, but fast enough to prevent the frames from blurring together.<ref>''Illuminating Engineering'' -- Illuminating Engineering Society 1954 Page 228</ref><ref>''Philips Technical Library - Fluorescent Lamps'' By J. L. Ouweltjes -- The MacMillan Press 1971 Page 32--40</ref> Even substances commonly associated with fluorescence may in fact be prone to phosphorescence, such as the liquid dyes found in [[highlighter]] pens, which is a common problem in liquid [[dye laser]]s. The onset of phosphorescence in this case can sometimes be reduced or delayed significantly by the use of triplet-quenching agents.<ref>''Principles of Lasers'' by Orazio Svelto -- Springer 2010</ref>

===Equation===
<math display=block>S_0 + h\nu \to S_1 \to T_1 \to S_0 + h\nu^\prime\ </math>
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.