Nonlinear Raman study of line shapes and relaxation of vibrational states of isotopically pure and mixed crystals of benzene

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Abstract
New experimental results on vibrational dynamics in crystalline benzene are presented. High resolution CARS measurements on a number of Raman active modes are presented for crystals of natural isotopic composition and for a 12C6H6 isotopically pure crystal. This is the first measurement of vibrational linewidths of an isotopically pure crystal. This study indicates that the naturally occurring 13C substituted isotopic impurities make a large quantitative contribution to the Raman linewidths. The residual low temperature linewidths of the pure crystal are consistent with mode specific energy relaxation. The effect of the 13C impurities on these lines is caused by near resonant scattering or trapping of vibrational excitons. In addition, the site energy dispersion due to disorder has been measured directly for the 991 cm−1 mode in a dilute mixed crystal. Also the relative contributions of scattering and trapping to the linewidths of the Ag and B2g factor group components of the 991 cm−1 band have been assessed.