Fast relaxation processes in glasses as revealed by depolarized light scattering

Applying tandem-Fabry-Perot interferometry together with a double monochromator, depolarized light scattering spectra were measured in order to investigate the fast relaxation processes and vibrations in molecular, ionic and polymeric glasses in the 1–5000 GHz range covering temperatures from the glass transition temperature Tg down to some 10 K. In addition to the boson peak, the spectra reveal quasi-elastic contributions that we attribute to (i) a nearly constant loss in the frequency range below ≅10 GHz and (ii) a power-law contribution with positive exponent α at higher frequencies. In the majority of glasses the latter may be attributed to thermally activated dynamics in asymmetric double well potentials as previously found for the light scattering spectra in silica. Following the Gilroy–Phillips model the exponent α shows a master curve as a function of T/V0 for the various glasses, where V0 specifies the width of the exponential distribution of barriers g(V), i.e., g(V) ∝ exp(−V/V0). The parameter V0 is found to be ∼Tg/2 in most cases. The relative strength of the dynamics in asymmetric double well potentials and the nearly constant loss contribution is different in the glasses studied.