The vibrations of vitreous silica observed in hyper-Raman scattering

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 SiOSi 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 SiOSi motions. On the opposite, the strong scattering from all longitudinal optic (LO) modes clearly follows the average isotropic medium 88m 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. Copyright (c) 2012 John Wiley & Sons, Ltd.