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°K. 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, ${\lambda }_c$, 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.