Raman scattering study of the dynamics of some fluorine compounds in the liquid phase

This manuscript deals with the application of various rotational and vibrational relaxation theories to the dynamic study of some high symmetry fluorine compounds. After a theoretical recall of the models we used, special attention is paid to the significance of the physical quantities derived from the Raman band contour analysis. Two types of movements are described: monomolecular ones and those involving many-body interactions. The first ones are studied through GORDON's and CHANDLER's models applied to NOF3, CF4 in Argon and SF6 in Oxygen and Krypton, and through DEBYE's model for the anisotropic PF3 molecule. A semiquantitative approach of the many-body interaction is given in the transition dipole - transition dipole coupling study of the highly infrared active modes in NOF3 and CF4. Generally, these theories are in fairly good agreement with experiment if one does not compare the results obtained on various compounds. Incoherencies may then appear between the values of the physical constants derived and those given in the literature. If the too small collision roughness of NOF3 can be explained by its peculiar type of atom bonding, the low reorientational anisotropy observed for PF3 enhances a major lack in DEBYE's model: it neglects interactions between neighbouring molecules of great importance in high density systems. CHANDLER's model is faulty in the case of SF6 dissolved in liquid Krypton because the size of Krypton no longer makes it a real hard sphere. To conclude we would say that experiment yielded results whose interpretation is no longer limited by the experimental quality but essentially by the over simplifications made in the models at our disposal. The appearance of new models based on numerical simulation gives a great hope for a better understanding of liquid dynamics. (author)