Low frequency light scattering in silica xerogels

New Brillouin and Raman scattering measurements on silica xerogels in a wide range of densities (0.5-2.2 g cm-3) are compared among each other and with data in melt quenched vitreous silica, in the 10-750 K temperature region. In the low frequency part of the Raman spectra, the boson peak characteristic of glasses is observed for all studied densities at low and high temperature when the quasielastic contribution is negligible. The results provide strong evidence of the presence of water in the pores of the silica skeleton.