Raman study of vibrational dephasing in liquid CH3CN and CD3CN

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Abstract
The Raman line shapes of the ν1(a1) C–H and C–D fundamentals in liquid acetonitrile and acetonitrile-d3 have been measured as a function of pressure up to 4 kbar within the temperature interval 30–120 °C. Densities have also been determined. From the isotropic component of the vibrational Raman band shape the vibrational relaxation times have been obtained as a function of temperature and pressure (density). The experimental results can be summarized as follows: (i) as T increases at constant density ρ, the vibrational relaxation rate (τvib)−1 increases; (ii) at constant T as density is raised τvib−1 increases; (iii) at constant pressure the T increase produces higher τvib−1, however, the change is more pronounced for the CD3CN liquid. Isotopic dilution studies of the CH3CN/CD3CN mixtures shows no significant effect on (τvib−1). The experimental data are interpreted in terms of the Kubo stochastic line shape theory and the dephasing model of Fischer and Laubereau. The results based on Kubo formalism indicate that dephasing is the dominant relaxation mechanism and that the modulation is fast. The isolated binary collision model proposed by Fischer and Laubereau for vibrational dephasing reproduces the essential features of the density and temperature dependence of the (τvib)−1 and suggests that pure dephasing is the dominant broadening mechanism for the isotropic line shapes studied. In the calculation the elastic collision times were approximated by the Enskog relaxation times.