In general phosphorescence is the long-lived emission of light which persists for longer than about one hundred nanoseconds after excitation. Shorter lived emission is termed fluorescence. The term is derived from the latin phosphorus-light bringing.
In molecular photochemistry, phosphorescence is defined as the emission of light associated with a transition between states of different spin multiplicity and usually arises from the transition from the first excited triplet to the ground state singlet. As discussed by Turro (1991) such transitions are "forbidden" and hence occur relatively slowly. Because of the long life of the triplet state the excited-state energy can be efficiently "quenched" in fluid solution, it is usual to work at 77 K in rigid organic glasses to observe phosphorescence. However in some cases, where molecules contain atoms other than those from the first row of the periodic table (e.g., halogenated polyaromatics and complexes of second or third row transition elements), room temperature phosphorescence can be detected. Oxygen is an efficient quencher of phosphorescence in fluid solution. Phosphorescence emission lies at lower energy than fluorescence. (See Photoluminescence for further details.)
The term phosphorescence is often used in solid state photophysics to indicate a temperature dependent emission process involving recombination of trapped electrons and holes (see also Thermoluminescence).
Turro, N. J. (1991) Modern Molecular Photochemistry, University Science Books, California.