Abstract
Thermal emission spectra of metal-backed thin films of BaF2 are observed at the emission angle of 25 degrees . Changes in the spectra are observed for the films prepared under different conditions. The spectra observed at elevated temperatures are analysed by using a response function based on the virtual-mode theory for an ionic film to estimate the temperature dependence of lattice dynamical quantities such as effective phonon frequency and damping. The damping around the longitudinal optical (LO) mode frequency is approximately proportional to temperature, while that around the transverse optical (TO) mode frequency varies non-linearly. The TO mode frequency decreases with temperature and the LO mode frequency is nearly the same for all temperatures. The experimental results are consistent with the existing theory of lattice anharmonicity. The frequency-dependent self-energy caused by the anharmonicity is significant for the temperature dependence of the phonon frequency and damping.
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