Temperature dependence of the vibrational relaxation time in liquid cyclohexane

The temperature dependence of the ultrasonic velocity and ultransonic damping as well as the hypersonic velocity and relaxation time τ_v1 was measured in liquid cyclohexane by Brillouin scattering. These data suggest a two-step relaxation process for the vibrational relaxing specific heat. According to the results at low temperature all but the two lowest vibrational modes, at high temperature all but the three lowest vibrational modes, are contributing to the specific heat of the first relaxation step. The relaxation time τ_v2 of the second step was calculated based on the bulk viscosity derived from the Brillouin line width. For the non-relaxing fraction of the bulk viscosity the corresponding values of Argon and Krypton using the theorem of the corresponding states were applied.

Finally the collision number Z₁₀ necessary for the deactivation of the lowest vibrational mode was evaluated on the basis of both the hard sphere model and the cell model. According to the Slawsky-Herzfeld theory a linear relationship between In Z₁₀ and T^−1/3 was found for both models.