Abstract
Polarised far-infrared spectral emission from KBr films is observed around the restrahlung band at an emission angle of 45 degrees . The spectra observed at elevated temperatures show an abnormal line shape around the longitudinal optical (LO) mode frequency. In order to discuss the effect of anharmonic phonon interaction on the spectra, these are analysed using a dielectric response function in which the empirical phonon self-energy function is considered. The results suggest that the shape of the damping function changes with temperature as well as with an increase of magnitude, which implies that the higher-order contribution to the self-energies becomes important at high temperatures as well as the lowest orders. The re-normalised quasi-harmonic LO mode frequency remains fairly constant close to the harmonic mode frequency for all temperatures. More than 10% of deviation from the Lyddane-Sachs-Teller relation is obtained at high temperatures.
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