Raman study of temperature and pressure effects on vibrational relaxation in liquid CHCl3 and CDCl3

The Raman line shapes of the ν₁(A ₁)C-H and C-D stretching fundamentals in liquid CHCl₃ and CDCl₃ have been measured as a function of pressure from 1 bar to 4·5 kbar within the temperature range 30°C to 90°C. Densities have also been determined under the same experimental conditions. The vibrational relaxation rates are obtained from the isotropic component of the Raman band and the experimental results can be summarized as follows: (i) as T increases at constant density the vibrational relaxation rate increases; (ii) at constant T, the increase in density produces an increase in the relaxation rate; (iii) an increase in temperature at constant pressure results in an increased relaxation rate. The above three cases hold for the CDCl₃ liquid, whereas only (ii) may be stated for the CHCl₃ liquid.

The experimental vibrational data are interpreted in terms of the Kubo stochastic line-shape theory and the collinear-isolated-binary-collision model proposed by Fischer and Laubereau. Application of the Kubo formalism shows that vibrational dephasing is the dominant relaxation mechanism and that the modulation is fast both in liquid CHCl₃ and CDCl₃.

Interpretation in terms of the binary collision dephasing model leads to the following results: (i) the pure dephasing mechanism seems to be the dominant broadening mechanism for the isotropic Raman line shapes studied; (ii) the calculated dephasing rates as a function of density and temperature show agreement with the experimental data. In these calculations the elastic collision times are obtained from the modified Enskog theory.