====== 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 Eu<sup>2+</sup> 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 R<sup>3+</sup> ions have been introduced into //M//Al<sub>2</sub>O<sub>4</sub>:Eu<sup>2+</sup> as co-dopants are either ignored or are contradictory. 
>
>\\ Source : [[https://www.sciencedirect.com/science/article/abs/pii/S0022459602001949|Persistent luminescence phenomena in materials doped with rare earth ions]] // Journal of Solid State Chemistry// 171 (2003) 114–122/

Further reading : [[https://en.wikipedia.org/wiki/Persistent_luminescence|Wikipedia]]

~~stars>1/5~~