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Optical properties of cesium iodide in the far infrared Beairsto, James Atley Bruce

Abstract

The reflectivity at room temperature and the absorption coefficient at room temperature, 77°K and 12°K have been measured for CsI. These and the results of a Kramers-Kronig analysis of the reflectivity by Vergnat et al. (1969) have been compared to the calculated optical constants (complex dielectric constant and complex refractive index), reflectivity and absorption coefficient. The calculation, using the lattice dynamical data of Karo and Hardy (1963), is based on the work of Wallis and Maradudin (1962) and Cowley (1963). The cubic coupling coefficient has been evaluated for nearest neighbors with a correction due to Eldridge (1973) for long-range Coulombic forces. The predicted features in the imaginary part of the complex phase shift, gamma, are all assigned to specific two-phonon processes except for a small feature at 91 cm⁻¹. The agreement between theory and experiment is very poor at room temperature but improves significantly in all cases at low temperatures. The discrepancies between theory and experiment give evidence of the fundamental importance of third and higher-order processes in the CsI lattice and suggest that next-nearest neighbor repulsive forces need to be included in the calculation of the cubic coupling coefficient.

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