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We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectraof amorphous silica. Model SiO2 glasses, uncompressed and compressed, weregenerated by a combination of classical and ab initio molecular-dynamicssimulations and their dynamical matrices were computed within the framework ofthe density functional theory. The Raman scattering intensities were determinedusing the bond-polarizability model and a good agreement with experimentalspectra was found. We confirm that the modes associated to the fourfold andthreefold rings produce most of the Raman intensity of the D1 and D2 peaks,respectively, in the VV Raman spectra. Modifications of the Raman spectra uponcompression are found to be in agreement with experimental data. We show thatthe modes associated to the fourfold rings still exist upon compression but donot produce a strong Raman intensity, whereas the ones associated to thethreefold rings do. This result strongly suggests that the area under the D1and D2 peaks is not directly proportional to the concentration of small ringsin amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres

Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical andab initiostudy