Free electrons, traps and glow in crystal phosphors

Abstract

The well known monomolecular and bimolecular theories of afterglow and thermal glow are summarized. Observed results on phosphors of ZnS type are discussed, particularly the phosphors with long afterglow in which the intensity-time laws are hyperbolic. These have been explained by many workers in this field by a monomolecular theory with a distribution of depths of electron traps. It is shown that certain mathematical approximations that must be used in relating the theory to observations of afterglow and thermal glow are not accurate enough. In part II a new and simple law is given that describes the same observations mathematically. In part III a new physical theory for the free and trapped electrons is outlined, and this is discussed in relation to F and F' centres in alkali halides and to an unsolved problem of semi-conductor theory.