Persistent luminescence

Persistent luminescence is the name given to the phenomenon found in materials which glow in the dark after the end of an excitation with UV or visible light - mainly phosphors such as silver-activated zinc sulphide or doped strontium aluminate. They typically glow a pale green to greenish-blue colour. They are routinely used in toys, watch dials, luminous paints, safety signs etc.

It has been known since the 1950s that the phenomenon involves so-called 'energy traps' (such as electron or 'hole' traps) 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/