On the theory of the far infrared absorption by alkali halide crystals

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Abstract
The infrared response of alkali halide crystals is investigated through a detailed analysis of the frequency dependent self energy of the transverse optic mode. Particular effort is made to understand the magnitude and temperature dependence of the width of the infrared absorption peak, given by the imaginary part of this self energy. The effects of intra-ionic nonlinearity are considered, and calculations are presented for KBr at 300 K indicating that these are small. The modification of the effective two-phonon density of states by phonon-phonon interaction is investigated, and calculations show that these can alter the self energy by up to 20%.