Infrared absorption due to multiphonon processes in the transparent regime of solids

Abstract
We have considered the problem of residual absorption in the transparent regime of a solid. In particular, the frequency dependence of absorption coefficient due to higher-order multiphonon processes in alkali halides has been treated. Joint phonon density of states up to sixth-order processes have been obtained using a breathing-shell model of lattice dynamics. Calculated values of absorption coefficient of NaCl and KCl agree with the observation of exponential frequency dependence in the transparent regime.