Raman and Rayleigh spectroscopy and molecular motions

Abstract

Extensive measurements of vibrational and, in a few cases, pure rotational Raman spectra of HCl, DCl, HBr and DBr dissolved in SF₆, C₂F₆, CCl₄ and SO₂ have been carried out at room temperature. Band profiles of both isotropic and anisotropic scattering have been studied and discussed in the light of existing theories on molecular motions in liquids. From isotropic scattering band-fitting it is concluded that, in the case of the three non-polar solvents, broadening due to vibrational fluctuations is small and that the band shape comes essentially from vibration-rotation coupling. Thus information about reorientational motions can be obtained directly from anisotropic scattering. For SO₂ solutions this statement is no longer valid because of the important contribution of vibrational fluctuations to the profile of the isotropic scattering. In this case the rotational correlation function can be determined, as for pure liquid hydrochloric and hydrobromic acids, by elimination of the vibrational correlation function from the Fourier transform of the anisotropic scattering. In the study of SF₆ and C₂F₆ solutions we have confirmed the existence of discrete rotational transitions in pure rotation or vibration-rotation spectra. Measured frequencies for J=2 to J=10 transitions show that perturbations of rotational levels from gas to solution are too small to be observed with our experimental accuracy.