Picosecond Measurements Of Relaxation Of Internal-Modes In Crystalline Benzene As A Function Of Temperature

The relaxation rates of four internal vibrational levels of benzene crystal have been measured by time-resolved coherent anti-Stokes Raman scattering (CARS) at different temperatures. The important information on the dephasing mechanisms provided by the experiment has been supported by anharmonic calculations, which include the full contribution of the density of phonon states, by utilizing average coupling coefficients. Different behaviors have been observed for the different modes considered. Three of the four vibrations (v1,v6, and v10) show linewidths that increase linearly in the classical regime with temperature; the experimental evidence for the important role played by three-phonon processes (driven by cubic anharmonicity) is confirmed by the calculations, which give a quantitative agreement with the observed linewidths. For the above-mentioned vibrations the role of pure dephasing results in a minor contribution, while the effect of isotopic impurities is important in determining the low temperature relaxation rate. On the other hand, the linewidth of v9 increases quadratically with T: Both decay processes of high order and pure dephasing may be responsible for such a behavior. This ambiguity cannot be overcome by our calculations, since these do not include the effect of high order (mainly quartic) anharmonic terms. Finally, the analysis of the decay mechanisms as predicted by the calculation shows that the anharmonic coefficients may differ from mode to mode: the relaxation mechanism is highly mode selective, and its efficiency depends greatly on the nature of the molecular normal coordinates involved in the process.