The nature and size of the luminescent centre

Abstract

There is considerable evidence that the luminescence of most inorganic solids due to the solid solution of a trace of an activator in another substance ( e. g. , Bi in CaS), and that absolutely pure salts do not show luminescence. Lenard has suggested that the absorbing and re-emitting centre in such cases consists of an atom or molecule of the activator linked with a definite number of the solvent molecules and attributes each luminescent band to a different centre. As to the nature of the association, there is evidence that it consists of the replacement of one of the metallic ions or atoms of the solvent lattice by one of the activating atoms. It has been shown for instance by Tiede and Schleede that luminescent solid solutions have a definite crystal structure whose lattice dimensions differ slightly form those of the pure solvent. Gruhl has produced samples of zinc silicate phosphors each showing a different luminescent band and each having a correspondingly different crystal structure.§ It is the object of this paper to show that simple assumptions in line with these ideas lead to a relation between brightness of luminescence and concentration of the activator which is in accord with experimental results and which gives an explanation of the existence of an optimum concentration. Further it will be shown that the experimental determination of this relation leads to a value of the number of molecules in a centre.