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Abstract
Using an interferometer-spectrometer in tandem with a limiting resolution of 0.005 cm-1 we have measured the band profiles of some low frequency Raman active phonons of monoclinic tetracyanoethylene (TCNE) and α-glycine at low temperature. In this latter case we obtained by deconvolution of the observed profiles residual bandwidths at T = 0 K as small as 0.004 cm-1 for two lattice phonons.The observed bandwiths are interpreted in terms of three-phonon decay processes and the most important relaxation mechanisms are determined. Correlation of the bandwidths with the one-and two-phonon density of states is used to localize the frequency range where decay into two acoustic phonons of the same energy is the dominant relaxation process.The present results are compared with those obtained previously for other molecular crystals such as naphthalene, anthracene and l-alanine. It is shown that at a given frequency the phonon bandwidths of the hydrogen bonded crystals are at least one order of magnitude smaller than those of the normal Van der Waals crystals. This effect is interpreted as due to the different anharmonicity of the intermolecular potential in the two cases.The evolution with temperature of the phonon bandwidths has been followed for α-glycine in the range 5-50 K. An excellent fit of the experimental data is obtained assuming a maximum of two preferential up-conversion decay mechanisms.
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