Isotropic Raman spectra of liquid (isotope) mixtures: Intermolecular coupling of vibrational modes

Abstract

The theory presented in this paper investigates the isotropic Raman spectra of liquid binary mixtures. It is found that the collective vibrational modes of different molecular species can be significantly coupled. This is a consequence of the (nearly) resonant vibrational transfer processes, which give rise to distinct vibrational correlations (i.e. correlations between adjacent molecules). The coupling, however, occurs only with weakly-separated or overlapping bands. The more general results of the theory are applied to isotope mixtures. The spectral information available from relevant dilution experiments is interpreted. In particular it is shown that the spectral properties of the vibrational self-correlation part can be concluded from the observed collective correlation function. The significance of the distinct vibrational correlations with respect to the infrared and depolarized Raman spectra is discussed.