A critical test of vibrational dephasing theories in solids using spontaneous Raman scattering in isotopically mixed crystals

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Abstract
A series of experiments have been conducted in order to evaluate the relative importance of several recent theories of vibrational dephasing in solids. The theories are discussed briefly, and are used to interpret the temperature dependence of the C–H and C–D stretch bands in the spontaneous Raman spectra of h 14‐ and d 14‐1,2,4,5‐tetramethyl benzene (durene). The infrared spectra of these same molecules are also reported in the region of the combination bands involving C–H (or C–D) stretches and low‐frequency modes. The results support the applicability of the model of Harris e t a l., [C. B. Harris, R. M. Shelby and P. A. Cornelius, Phys. Rev. Lett. 38, 1415 (1977); Chem Phys. Lett. 57, 8 (1978); R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem. Phys. 70, 34 (1979)], based on energy exchange in anharmonically coupled low‐frequency modes. This theory is then used, in connection with Raman spectra obtained in isotopically mixed samples of durene, to elucidate the vibrational dynamics underlying the dephasing. It is found that the results are consistent with the hypothesis that some low‐frequency modes in this molecule are significantly delocalized or ’’excitonic’’ in character, and that this delocalization may be studied by means of Raman spectroscopy on the low‐frequency modes themselves, as well as by exchange analysis of the coupled high‐frequency modes. These conclusions represent a generalization and extension of the previously published exchange model [R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem Phys. 70, 34 (1979)].