Résumé
The concentration of the fourfold and threefold ring modes of normal and densified silica has been quantitatively extracted using time-domain Raman spectroscopy. Their frequency and width are precisely determined even when they are partly masked by a broad background in spontaneous Raman spectroscopy [1,2]. Experiments performed in GeO2 also reveal the existence of a weak vibrational mode ascribed to oxygen motion in three-membered rings [3]. In permanently densified silicas, the variation of the Raman intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient CB µ w2. This allows to derive a simple law relating the frequency of the bending modes with the Si-O-Si bond-angle in the network and in the fourfold and threefold rings [4]. The model has been applied to densified silicas and sodo-silicates glasses, and the results compare well to numerical simulations. This simple Raman-spectra analysis could become part of the routine characterization of the local and medium range structure of silica-based glasses.