Far-infrared absorption of alkali halide crystals containing H− ions

Abstract

The optical absorption from localized vibrations associated with the H− ion as a substitutional impurity in KBr and KI is investigated in the far infrared. A force constant model for the defect is derived from a theoretical fit of the experimental results for the anharmonic side band of the H− local mode. This model is used to predict the absorption spectrum of the defect in the far infrared below the Reststrahlen frequency. Good agreement is obtained with absorption measurements at low temperature. It is shown that some of the observed peaks correspond to localized modes, while others are directly related to van Hove singularities in the frequency spectrum of the host crystal.