Second-order Raman scattering in non-crystalline materials: Amorphous Se

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Abstract
Second-order Raman scattering is reported for the high-frequency optic-like modes in bulk amorphous Se. The results are analysed in terms of the general role of crystalline momentum relaxation in second-order processes in non-crystalline materials. It is shown that k relaxation yields additional combination processes that result in general in a weighted self-convolution of the one-phonon density of states. The form of the second-order spectra in amorphous Se suggests, however, that the coupling parameters for combination bands are reduced relative to overtone contributions. The predominance of second-order overtone processes allows an improved estimate of the one-phonon density of states for the high-frequency modes in amorphous Se. The phonon density of states has been employed to estimate the frequency variation of the first-order Raman and I.R. coupling parameters for the full spectral range.