Law of conservation of momentum and rule of mutual exclusion for vibrational excitations in hyper-Raman and Raman spectra of glasses

Abstract

The dependences of hyper-Raman and Raman spectra on the scattering angles and polarization are studied in SiO 2, TiO 2SiO 2 and the other oxide glasses. It is found that the vibrational excitations (both polaritons and purely mechanical vibrations) are characterized by well-defined wave vectors in these glasses. The absence of polaritons in the Raman spectra and the band frequency noncoincidence in the hyper-Raman and Raman spectra have allowed us to come to the conclusion that polar modes of glasses were forbidden in the Raman effect (the rule of mutual exclusion). A new approach to the interpretation of vibrational spectra in glasses is suggested. In accordance with this approach the spectra of glasses must consist of two parts: the first one (disordered) is proportional to the density of vibrational states, the second one is crystalline-like and satisfies the selection rules including the law of conservation of momentum for the isotropic medium of the ∞ ∞ m symmetry. The frequencies of the intensity maxima in the hyper-Raman spectra are shown to be related to the crystalline-like part.