Intensity perturbations from vibrational resonance coupling in isotropic Raman spectra of neat liquids and solutions

The influence of vibrational resonance coupling (VRC) on the intensity of isotropic Raman spectra is investigated theoretically for neat liquids and binary mixtures. Experimental studies of the effect are presented for overlapping Raman bands in binary liquid solutions (DMSO/CHCl3 and DMSO/CDCl3) perturbed by hydrogen bonding between the two components. The vibrational resonance coupling perturbation leads to a nonlinear density dependence of the intensity, and is strongly dependent on the presence of short range order. An estimate of the typical magnitude of the intensity perturbation due to VRC is made by assuming that the coupling derives from the interaction of vibrational transition dipoles. It is concluded that the intensity perturbations are ordinarily quite small except in the case of low frequency vibrations having large transition dipoles, or in the case of very strong orientational correlations. For VRC between vibrations localized,on molecules of different species, the role of the frequency matching is explored theoretically and experimentally, and it is found that the frequency mismatch must to be small compared to thermal energy kT for appreciable intensity perturbations to be observed. (C) 1997 American Institute of Physics.