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Phosphorescence is a type of photoluminescence related to fluorescence, but unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur very slowly in certain materials, absorbed radiation is re-emitted at a lower intensity for up to several hours after the original excitation.
Phosphorescence vs chemiluminescence
Some examples of glow-in-the-dark materials do not glow by phosphorescence. For example, glow sticks glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemiluminescence, an excited state is created via a chemical reaction. The light emission tracks the kinetic progress of the underlying chemical reaction. The excited state will then transfer to a dye molecule, also known as a sensitizer or fluorophor, and subsequently fluoresce back to the ground state.
Common phosphorescent materials
- Calcium sulfide
- Strontium aluminate
- Zinc sulfide