Persistent luminescence is the phenomenon found in materials which glow in the dark after the end of an excitation with UV or visible light. There are a number of such materials, routinely used in toys, watch dials, luminous paints, safety signs etc. They are mainly phosphors such as silver-activated zinc sulfide or doped strontium aluminate, and typically glows a pale green to greenish-blue color.
It has been known since the 1950s that the phenomenon involves so-called 'energy traps' (such as electron or hole trap) in a material, but the an exact explanation of the underlying physics is still lacking.
The overall mechanism of the persistent luminescence is now quite well agreed on to involve the formation of traps followed by a subsequent thermal bleaching of the traps and emission from the Eu2+ sites. Despite the seemingly simple stoichiometry and structure of the alkaline earth aluminates, the determination of persistent luminescence mechanisms seems to present a very complicated problem. Accordingly, no general agreement has been achieved on the detailed mechanisms involved and several interesting and even exciting mechanisms have been proposed. Especially, the mechanisms resulting in the prolonged and enhanced afterglow when R3+ ions have been introduced into MAl2O4:Eu2+ as co-dopants are either ignored or are contradictory.
Source: Persistent luminescence phenomena in materials doped with rare earth ions Journal of Solid State Chemistry 171 (2003) 114–122/
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