The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica

Spectroscopic results on low-frequency excitations of densified silica are presented and related to characteristic thermal properties of glasses. The end of the longitudinal acoustic branch is marked by a rapid increase of the Brillouin linewidth with the scattering vector. This rapid growth saturates at a crossover frequency ?co which nearly coincides with the centre of the boson peak. The latter is clearly due to additional optic-like excitations related to nearly rigid SiO4 librations as indicated by hyper-Raman scattering. Whether the onset of strong scattering is best described as resulting from hybridization of acoustic modes with these librations, from their elastic scattering (Rayleigh scattering) on the local excitations, or from soft potentials remains to be established.