Fluorescence vs Phosphorescence

Usually re-emission of radiation, if it occurs, is within nanoseconds (10-9 sec)of the excitation= FLUORESCENCE. PHOSPHORESCENCE is a much slower (and rarer) process. After source of excitation radiation is switched off, phosphorescence emission continues for periods that vary from milliseconds to weeks. Phosphorescence is caused by electron becoming transferred into a triplet state. (Electrons of the same spin in the one orbital). Triplet states have long lifetime so phosphoresence persists. Mostly requires low temperatures (<<0oC) - at higher T, triplet state is de-activated by quenching.