Abstract
Hyper-Raman scattering (HRS) is performed in v-SiO2, and the results are compared to RS and infrared absorption data. Mode analyses are tested based on two models, either the local C2v symmetry of the Si-O-Si bonds or the Td symmetry of the SiO4 tetrahedra. The HRS boson-peak (BP) signal is dominated by the quasi-local librations of tetrahedra. However, we find that it also contains a weak dipolar contribution with a different spectral signature, suggesting that the BP modes are not fully hybridized. The transverse optic (TO) modes can be assigned to molecular-like excitations according to either model, with the exception of the lowest frequency TO4 which involves highly cooperative Si-O-Si motions. On the opposite, the strong scattering from all longitudinal optic (LO) modes clearly follows the average isotropic medium ∞∞m selection rules. This long range cooperativity presumably results from the associated internal electric field. Finally, the strong LO HRS signals enabled observing a new longitudinal component at 1078 cm−1. Through the example of v-SiO2, the article also highlights new perspectives provided by HRS for the description of the vibrations in disordered materials in general.