Phosphorescence is a relatively long-lived light emission from a material. The phosphorescence lifetime is significantly longer than the fluorescence lifetime of the material. It can occur when an electron decays from a higher electronic energy state of an atom, ion or molecule to an unfilled lower energy state. In many cases, the higher energy level for phosphorescence emission is a triplet state. The electronic transitions that result in phosphorescence emission are sometimes called ‘forbidden’ transitions. ‘Forbidden’ transitions occur at much slower speeds than ‘allowed’ transitions. A typical phosphorescence lifetime of a material can be milliseconds or even seconds, whereas the fluorescence lifetime of a material is usually measured in nanoseconds.

The diagram below shows phosphorescence emission resulted from radiative relaxation of an electron from electronic triplet state to the ground electronic singlet state of a material. Vibrational states are not shown in the diagram.

radiative relaxation

See also: Fluorescence, Relaxation.

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