Studies of molecular motions of CH2Br2 in the liquid state by depolarized Rayleigh and Raman scattering

We have obtained from the concentration dependent depolarized Rayleigh scattering experiment (using a Fabry–Perot interferometer) the single particle reorientation relaxation time τs of CH2Br2 to be 3.4±0.2 psec at 20 °C. This time is a factor of 2.0 greater than the Raman correlation time τm obtained for the ν2(A1) Raman band. Moreover, the Raman correlation times τm obtained for the other A1 bands do not have the same values as that obtained for the ν2 band. This difference is due to the fact that the nonreorientational correlation functions Ciso(t) and Cβ(t) do not have the same time dependence, hence making the usual Raman line shape analysis difficult to apply to CH2Br2. From depolarized Rayleigh scattering, we have found in neat CH2Br2 that the classical Stokes–Einstein relation for rotational diffusion does not apply, despite the fact that τRay was found to vary linearly with η/T. However, when using the slip boundary condition and modeling CH2Br2 as an ellipsoid, we have found reasonable agreement between theory and experiment. This work is the first temperature dependent study of molecular motion in simple liquids involving the joint use of the Rayleigh and Raman scattering techiques. We have shown that by combining results from temperature and concentration dependent Raman and depolarized scattering experiments, significantly more information concerning molecular motions can be obtained than is possible from either technique alone.