A phenomenon related to fluorescence is phosphorescence, which is the
emission of light following intersystem crossing between electron orbitals (e.g.
between excited singlet and triplet states). Light emission in phosphorescence
usually continues after the exciting energy is no longer applied and, since
more energy is lost in intersystem crossing, the emission wavelengths are
generally longer than with fluorescence. Phosphorescence has limited applications
in chemical sciences.
Luminescence (or chemiluminescence) is another phenomenon in which
light is emitted, but here the energy for the initial excitation of electrons is
provided by a chemical reaction rather than by electromagnetic radiation. An
example is the action of the enzyme luciferase, extracted from fireflies, which
catalyses the following reaction:
|⇒ Equation [26.6]
||luciferin + ATP + O2 → oxyluciferin + AMP + PPi + CO2 + light
The light produced is either yellow-green (560nm) or red (620nm). This
system can be used in biomolecular analysis of ATP, e.g. to determine ATP
concentration in a biological sample. Measurement can be performed using
the photomultiplier tubes of a scintillation counter to detect the
emitted light, with calibration of the output using a series of standards of
known ATP content.