Raman band shapes and dynamics of molecular motion for SF6 in the supercritical dense fluid region

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Abstract
The Raman reorientational correlation functions and the reorientational correlation times τϑ for the ν1, ν2, and ν5 bands of SF6 have been obtained in the temperature range 253 – 473 °K and at pressures up to 1865 bar. The plot of τϑ(Raman) vs τJ (obtained from the NMR relaxation times) clearly indicates that the reorientational relaxation of SF6 follows the Fokker–Planck–Langevin (FPL) model in the low density region. The experimental correlation functions were compared with correlation functions computed with the extended diffusion (ED) model and the FPL model. Both models reproduce the experimental correlation functions with the same accuracy; the values of angular momentum correlation times τJ used to produce the functions are approximately the same and close to the τJ values obtained from NMR 19F spin–lattice relaxation times. However, the fitting procedure, i.e., comparison of experimental correlation functions with the theoretical ones, does not allow an unambiguous conclusion as to which of the models describes the molecular reorientation of SF6. Another important finding of this study is the observation of a nonlinear behavior of the plot of τϑ vs η/T (η is the viscosity) Hynes et al. Detailed analysis of the ν1 band is presented and it is shown that vibrational–rotational coupling is responsible for the ν1 band shape.