Low frequency dynamics of silica xerogel porous system

Coherent inelastic neutron and light (Raman and Brillouin) scattering measu rements 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 presenc e of the pores and its frequency connected to their mean sizes. This import ant result seems to indicate that the disorder introduced by the presence o f the pores does not influence very much the density of states in the frequ ency 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 den sity and temperature. The absence of a peak in the inelastic structure fact or for the Q value corresponding to the first sharp diffraction peak demons trates the existence of random-phase modes for energies around the boson pe ak.