Inelastic neutron scattering on silica xerogel porous systems

New coherent inelastic neutron scattering measurements on silica xerogels in the 300–1000 K temperature range are presented and compared with those obtained in vitreous silica (heralux). The spectra show the presence of the Boson peak, at temperatures, where the quasi-elastic contribution due to absorbed water is negligible. The Boson peak in silica xerogels is at a frequency of around 5 meV (40 cm⁻¹) and has a shape similar to that of melted silica, even in the smaller density xerogels. This important result seems to indicate that the disorder introduced by the presence of pores does not affect the density of states in frequency region. This evidence is also confirmed by changes in the inelastic structure factor S(Q,ω), measured at the frequencies of the Boson peak as a function of the exchanged Q.