Light-scattering spectra of fast relaxation in glasses

Light-scattering spectra of the glasses polystyrene (PS), polycarbonate (PC ), and Ca0.4K0.6(NO3)(1.4) (CKN) are measured in the frequency interval 3 - 10000 GHz covering a broad temperature range. The low frequency wing of th e fast relaxation spectrum is found to show a power-law behavior with an ex ponent alpha= 0.2- 0.6. 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 dou ble-well potentials well describes the power-law parr of fast relaxation sp ectra in PS and CKN but fails in the case of PC. The distribution of barrie r heights is extracted from the spectra. [S0163-1829(98)04645-1].