Frequency spectra and normal modes of vibration have been computed for vitreous silica. They have been calculated from atomic arrangements in physical models based on the random network theory. The positions of bands in the computed spectra agree well with observed features in the experimental infra-red and Raman spectra of the glass. Detailed analysis of the normal modes indicates that the bands at 1050, 750 and 400 cm–1 are associated with bond-stretching, bending and rocking motions, respectively, of the oxygen atoms. Atomic vibrations in the glass are, on the whole, less extended in space than the plane wave-like modes which prevail in perfect crystals. The spatial localization tends to be greatest at high frequencies and near band edges. If non-bridging oxygen atoms are present in the structure, the frequenncy spectrum exhibits an additional band of very intense localization, associated with bond-stretching vibrations of the non-bridging atoms.
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