Far-Infrared Measurements of the Optical Properties of CsI between 12 and 300 °K, Compared with Calculations Based on Cubic Anharmonicity Only

Abstract

Measurements of the absorption coefficient of bulk CsI from 15 to 60 and from 90 to 200 cm⁻¹ at 12, 77, and 300 °K, together with the normal reflectivity at 300 °K, are reported. These have been compared with calculations based on cubic anharmonicity, which involved the derivation of the cubic coupling coefficient for the CsI lattice. From inspection of this coefficient, certain selection rules have been obtained for the two-phonon relaxation of the transverse-optic resonance. These rules and other criteria, when applied to CsI, predict just two main features, due to the summation processes from R₂₅′R₁₅ and X₅X₅′. The agreement in intensity at low temperatures is very good, and confirms the need to include next-nearest neighbors in the lattice repulsive potential. Wavenumber discrepancies of the features appear to be a result of inaccuracies in the deformation-dipole input data. By 300 °K the need for quartic anharmonicity is evident both in the resonance region and at high wavenumbers. Portions of the damping spectra from these terms have been obtained and are discussed.