Raman Study of Liquids. I. Theory of the Raman Spectra of Diatomic Molecules in Inert Solutions

A theory is proposed to explain the Raman spectra of inert solutions of diatomic molecules; this theory can also be considered as an approximate theory of Raman spectra of pure liquids. The theory is a stochastic-type theory related to similar theories of infrared and NMR spectra. The vapor-solution band shifts are shown to depend on the difference between the solvent-solute interaction energies of the two vibrational states involved in the transition. The analysis of the band profiles is made separately for the isotropic and anisotropic components, respectively, of a Raman spectrum. The former spectrum is produced by vibrational relaxation mechanisms alone; the bands are all asymmetric although, in certain cases, the asymmetric perturbation is small enough to be neglected. The latter spectrum is produced by both vibrational and reorientational relaxation mechanisms. The theory predicts the existence of a continuous sequence of band forms comprising, among others, the profile with an $O\ensuremath{-}Q\ensuremath{-}S\ensuremath{-}$ type structure, the Lorentzian profile, the Gaussian profile, the Voigt profile, and several sorts of asymmetric profiles. The resulting Raman spectrum appears as a superposition, with appropriate coefficients, of an isotropic and anisotropic spectrum. A procedure is indicated permitting a separate study of vibrational and reorientational relaxation effects.

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