Résumé
Hyper-Raman scattering from the Boson peak has been performed in vitreous silica and in a silica-based lead glass. A comparison with Raman scattering and infrared measurements suggests that in v-SiO
2 the low-
ω excitations are associated with rotational motions of SiO
4 tetrahedra. These modes dominate the vibrational density of states in that frequency range. In the lead glass, despite a strong modification of the SiO
4 network as compared to pure silica, the results indicate that the low-frequency density of states might also be dominated by hyper-Raman active modes.