Low frequency dynamics of silica xerogel porous system

Complete text of publication follows. Coherent inelastic neutron (INS) and light scattering measurements on silica xelogels over a wide range of densities and temperatures are presented Raman scattering (RS) results show the presence of a Boson Peak (BP) at temperatures where the quasielastic (QES) contribution is negligible. The BP of xerogels is at a frequency around 35 cm-1 and has a shape similar to that of melted silica. Another bump at 7 cm-1 is also visible in RS, which is connected to the presence of the pores. While BP is also revealed by INS, this second peak is not. This important result seems to indicate that the disorder introduced by the presence of the pores doesn't influence the density of states, and therefore is not seen by neutrons, but only by RS (apparently sensitive to details of the morphology). A comparison between the Q-dependence of the elastic and inelastic structure factors as a function of sample density and temperature is also presented. 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 BP. (author)