Phosphorescence and luminescence

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.