Far-Infrared Optical Properties of KCl and KBr

Abstract

The far-infrared optical properties of KCl and KBr have been measured at $T=300\mathrm{}°$K by the method of asymmetric Fourier-transform spectroscopy. This method allows the simultaneous measurement of the amplitude and phase shifts of the radiation reflected or transmitted by the crystals. From reflectance data, the real and imaginary parts of the complex index of refraction were obtained in the spectral region from approximately 40 to 360 ${\mathrm{cm}}^{-1\mathrm{}}$. Detailed numerical calculations for KCl were performed by employing a model in which the radiation was assumed to interact with the k=0 TO phonon mode via the first-order dipole moment and the subsequent decay of this mode by two-phonon processes. The frequency-dependent Hermitian and anti-Hermitian parts of the k=0 self-energy, arising from the cubic-lattice anharmonicity, were calculated by using the phonon frequencies and eigenvectors obtained from the Karo-Hardy deformation-dipole model. The complex index of refraction of KCl was calculated from the self-energy and the results are in resonable agreement with experiment. Finally, a comparison is then made for KBr between the calculations performed by Cowley and our experimental data.