VIBRATION ANHARMONICITY AND PICOSECOND RELAXATION IN GLASSES

It is shown that vibration anharmonicity can be responsible for the qu asielastic line in the low-frequency Raman spectra of glasses. The mec hanism is based on the damping of vibrations by the phonon density flu ctuations. They are coupled to vibrations due to third-order anharmoni city. This coupling produces the quasielastic response with a characte ristic width determined by the relaxation time of the phonon density f luctuations, which is of the order of a picosecond. It is shown that t he ratio of the integral quasielastic to vibration spectrum is determi ned by the Gruneisen parameter squared. Comparison with experimental d ata shows that this mechanism gives the dominant contribution to the q uasielastic Raman scattering in glasses and supercooled liquids near t he glass transition temperature. It is shown that in various glasses t he inverse relaxation time linearly correlates with the frequency of t he boson peak, in good agreement with an empirical observation.