Raman Studies of Reorientational and Vibrational Relaxation in Dense Fluids
Perry, Steven James
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Permalink
https://hdl.handle.net/2142/70166
Description
Title
Raman Studies of Reorientational and Vibrational Relaxation in Dense Fluids
Author(s)
Perry, Steven James
Issue Date
1981
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Abstract
Raman spectroscopy can provide much information about the relaxation processes in dense fluids. This thesis consists of studies of the reorientational relaxation in tetrafluoromethane and propyne, and of the vibrational relaxation in propyne and group IV tetramethyl compounds.
The anisotropic Raman spectrum of the (nu)(,2)(E) band at 439 cm('-1) in CF(,4) has been measured as a function of temperature and pressure. It is shown that the reorientational motion of CF(,4) in the dense fluid and moderately dense gas region is best described by the Fokker-Plank-Langevin model rather than the Extended J-diffusion model. Two methods of testing reorientational models are discussed. The first method, consisting of the independent measurement of the reorientational correlation time and the angular momentum correlation time, and comparing the quantities to the model-dependent relationships is shown to lead to definite conclusions regarding the reorientational motion. The second method, consisting of comparing model-dependent reorientational correlation functions with experimental ones is shown to be an insensitive test and leads to ambiguous results.
The reorientational motion of propyne in the liquid phase and in solutions with inert and active solvents is reported as a function of temperature and pressure. An increase of the reorientational correlation time with increasing pressure has been found. It is shown that the theoretical correlation functions for diffusion-type models (the extended diffusion theory and the Fokker-Planck-Langevin friction theory) can reproduce the experimental correlation functions of propyne only in the absence of hydrogen bonding. It is also shown that a relatively free rotation of the propyne symmetry axis in acetone-propyne complexes takes place even at the highest pressures. Two simple pictures of this motion are proposed.
The vibrational relaxation in propyne complexed with acetone has been studied as a function of temperature and pressure. The experimentally determined correlation functions have been calculated. It is shown that these functions have a long time tail which is negative. This effect is explained in terms of a slowly relaxing rotational motion which causes the environment of the scattering propyne molecule to change on a time scale comparable to the vibrational relaxation time. The short time portion of the function is shown to be dominated by some type of collision effect. The vibrational relaxation of propyne in inert solutions has also been studied and the results indicate that the band broadening is in the "motionally narrowed" limit.
The vibrational relaxation in group IV tetramethyl compounds has been studied for the (nu)(,1) and (nu)(,3) bands. The (nu)(,1) bands are shown to be broadened by Fermi resonance effects. The vibrational relaxation process for the (nu)(,3) mode is interpreted in terms of the binary collision model of Fischer and Laubereau.
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