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 liq uids and binary mixtures. Experimental studies of the effect are prese nted for overlapping Raman bands in binary liquid solutions (DMSO/CHCl 3 and DMSO/CDCl3) perturbed by hydrogen bonding between the two compon ents. The vibrational resonance coupling perturbation leads to a nonli near density dependence of the intensity, and is strongly dependent on the presence of short range order. An estimate of the typical magnitu de of the intensity perturbation due to VRC is made by assuming that t he coupling derives from the interaction of vibrational transition dip oles. It is concluded that the intensity perturbations are ordinarily quite small except in the case of low frequency vibrations having larg e transition dipoles, or in the case of very strong orientational corr elations. For VRC between vibrations localized,on molecules of differe nt species, the role of the frequency matching is explored theoretical ly and experimentally, and it is found that the frequency mismatch mus t to be small compared to thermal energy kT for appreciable intensity perturbations to be observed. (C) 1997 American Institute of Physics.