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Abstract

Measurements and calculations of the far-infrared optical properties of K ³⁹I at 300, 77, and 12*K are presented. The measurements are mainly those of absorption in crystals of various thicknesses. The calculation assumed cubic anharmonicity only, with nearest-neighbour coupling, and the input lattice-dynamlcal data were obtained from a shell-model program. These data were generated with a wave vector density of 32000 points per zone, which, was sufficient to give 2-3 cm⁻¹ resolution. The over-all agreement between experiment and theory, in both: the intensity and structure of the spectra, is good. The magnitude of certain calculated features is, however, too large, indicating a need to consider next-nearest-neighbour interactions. Evidence was also found for three-phonon damping, both beyond the two-phonon limit at all temperatures and at J[sub o] by 300°K. From these measurements it was possible to calculate portions of the three-phonon damping spectra, which were found to be reasonable. The higher-phonon effects at 300°K did not seem to be noticeably more pronounced than those found in the much harder LiF, and arguments are presented to understand this. Finally, the isotope-induced one-phonon processes which occur in natural KI were calculated. These are shown to be small away from the resonance frequency v, and not to be the major damping mechanism at V[sub e] at low, temperatures^ in contrast to Lif.