Multiphonon absorption by ionic crystals: Temperature dependence

Abstract

We consider the part of the temperature dependence of the multiphonon absorption in the ir that is not dependent on the mechanism from which the absorption arises. Several phonons must act together to provide absorption at high frequencies; the availability of each phonon alone provides a temperature dependence that is the same whatever the mechanism for coupling them together may be. These availability factors must then be summed over the existing frequency distribution in a particular crystal, subject to the condition that the sum of frequencies equal that of the incoming light. Numerical results are given for KC1 and NaC1 at 10.6 μ. We find the temperature dependence to be different depending on whether the incoming photon couples to a lattice or an impurity phonon of higher frequency. This suggests experiments that could reveal whether the residual absorption found in nominally ``pure'' crystals is in fact intrinsic or due to unknown impurities.