Temperature dependent spectral line shifts in crystalline and polycrystalline materials

Abstract

The temperature dependence of optical window materials remains an important issue for the windows and domes community. Concerning the refractive index, current models are empirically based polynomial fits for the Sellmeier model strength and mode location parameters making it impossible to accurately extrapolate beyond the measured temperature range. Thus, the development of a theoretical model for spectral line shifts as a function of temperature is an important goal. Such a model will allow extrapolation as long as the physical mechanisms don’t change. For vibrational modes, a thermal average of the anharmonically shifted energy levels will be investigated and compared to experimental data. The first anharmonic term can be estimated using the Morse potential based multiphonon absorption model. Experimentally the modes red shift which is consistent with what the proposed model should predict.