On the luminescence and absence of luminescence of F centers

A model, proposed originally by Dexter, Klick and Russell, is used to explain the occurence or non-occurence of F-center luminescence in ionic crystals, in terms of a simple two-state configuration coordinate diagram. In this model, which works for all known cases, luminescence is quenched by a competing non-radiative process whenever the intersection of the ground and excited state curves lies below the energy reached in absorption in a vertical (Franck-Condon) transition. The criterion for the occurence of luminescence is expressed as $\text{Λ <}\text{1}\text{4}$, where Λ (=excited-state lattice-relaxation energy/optical-absorption energy) is a parameter, related to the relative displacement of the two curves, which can be inferred from data on the temperature dependence of the F-band line width. Thus the possibility of observing luminescence can be predicted from optical absorption data alone. It is found emperically that Λ for alkali halides with rocksalt structure is independent of lattice parameter, and the observed dependence of Λ on the ratio of ionic radii in terms of ion-size effects. Values of Λ range from 0.009 for CsF to 0.831 for LiI; NaCl with Λ = 0.260 is a marginal case for luminescence.