Résumé
The hyper-Raman spectroscopy (HRS) of vitreous silica is performed. It shows that longitudinal optic modes fulfill the isotropic media selection rules, emphasizing their delocalized character. This behavior likely originates from their electro-optic coupling with the macroscopic electric field. More surprisingly, the transverse optic (TO) vibration of lowest frequency has intermediate HRS scattering strengths, between local and averave media selection rules. This has been assigned to a strongly delocalized charater of the mode, a behavior not often observed in glasses.