Far-Infrared Lattice Absorption in Alkali Halide Crystals

Measurements of the absorption between 0. 3 and 1. 0 mm have been made on NaCl, KCl, KBr, KI, NaI, and LiF between room temperature and 15^. Present theory explains the absorption in this region as arising from "vertical" transitions from one phonon state to another with the same wave vector but of a higher energy, the energy difference between the two states being the energy of the absorbed photon. At high temperatures such 2-phonon processes should generally lead to a linear temperature dependence of the absorption. Experimentally, however, we found in all cases a strong additional contribution to the absorption which increased as T^2, which we believe to be caused by 3-phonon difference processes. At wavelengths longer than a certain cutoff wavelength, ₂, depending on the dispersion curves, the linear (2-phonon) contribution disappears, leaving only the T^2 contribution. Comparison of our results with phonon dispersion curves shows that 2-phonon processes involving one longitudinal and one transverse branch do not appear. The results suggest that the observed 2-phonon absorption arises only from transitions between branches of the same polarization.