Low frequency dynamics of silica xerogel porous system

Coherent inelastic neutron and light (Raman and Brillouin) scattering measurements are reported on silica xerogels over a wide range of densities and temperatures. Raman scattering results show the presence of a boson peak at temperatures where the quasielastic contribution is negligible. The boson peak of xerogels is at a frequency around 35 cm(-1) and has a shape similar to that of melted silica. At lower frequencies (7-16 cm(-1)) another bump is visible in the Raman scattering, believed to be connected to the presence of the pores and its frequency connected to their mean sizes. This important result seems to indicate that the disorder introduced by the presence of the pores does not influence very much the density of states in the frequency range of the boson peak. A comparison is made between the Q dependence of the elastic and inelastic structure factors as a function of sample density and temperature. The absence of a peak in the inelastic structure factor for the Q value corresponding to the first sharp diffraction peak demonstrates the existence of random-phase modes for energies around the boson peak.