Abstract
Spectroscopic results on low-frequency excitations of densified silica arepresented and related to characteristic thermal properties of glasses.The end of the longitudinal acoustic branch is marked by a rapidincrease of the Brillouin linewidth with the scattering vector. Thisrapid growth saturates at a crossover frequency Ωcowhich nearly coincides with the centre of the boson peak. The latter is clearlydue to additional optic-like excitations related to nearly rigid SiO4librations as indicated by hyper-Raman scattering. Whether the onset of strongscattering is best described as resulting from hybridization of acoustic modes withthese librations, from their elastic scattering (Rayleigh scattering) on thelocal excitations, or from soft potentials remains to be established.
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