Light-scattering spectra of fast relaxation in glasses

Light-scattering spectra of the glasses polystyrene (PS), polycarbonate (PC), and (Formula presented) (CKN) are measured in the frequency interval (Formula presented) covering a broad temperature range. The low frequency wing of the fast relaxation spectrum is found to show a power-law behavior with an exponent (Formula presented) The exponent depends on system and temperature. No indication of a crossover to a white noise spectrum, as previously reported and discussed within mode-coupling analyses is found. It is shown that the Gilroy-Phillips model of thermally activated transitions in asymmetric double-well potentials well describes the power-law part of fast relaxation spectra in PS and CKN but fails in the case of PC. The distribution of barrier heights is extracted from the spectra.