Spectral signature of low temperature hopping between two impurity-induced elastic configurations.

We demonstrate that the jump-rotational-diffusion model originally developed in the context of quasielastic neutron scattering from high temperature liquids can describe the two-state behavior observed in the low temperature dynamics of some molecular and point defects in crystals. The unusual temperature dependences of the IR spectra of both diatomic chalcogen hydrides in alkali halides and KI : Ag can be explained in terms of two elastic configurations between which the impurities jump with temperature-dependent dwell times.