Isotropic and anisotropic intermolecular dynamics of liquids studied by femtosecond position-sensitive Kerr lens spectroscopy

Femtosecond Fourier-transform position-sensitive Ken lens spectroscopy is shown to be a powerful technique for obtaining the isotropic and anisotropic components of the low-frequency, intermolecular Raman spectra of liquids. The isotropic and anisotropic spectral features are compared for liquids benzene, toluene, and benzonitrile in order to characterize the intermolecular motions in terms of their relative contribution to the different elements of the Raman susceptibility. We observe that the spectral profiles for the isotropic and anisotropic components of the Raman susceptibility tensor of both benzene and toluene are identical within our experimental uncertainty. A frequency-independent depolarization ratio of 0.7+/- 0.1 and 0.75+/-0.1 are obtained throughout the 0-150 cm(-1) region, respectively, for benzene and toluene, respectively. This ratio indicates that the collective intermolecular vibrations can be described as being ''depolarized.'' On the other hand, we observe that the intermolecular depolarization ratio obtained for benzonitrile is frequency-dependent, gradually decreasing from 0.7 near 0 cm(-1) to approximately 0.5 at 150 cm(-1). This observation indicates that the intermolecular interactions in benzonitrile become increasingly ''polarized'' as the frequency is increased. These data are discussed in terms of the distinct molecular properties of each liquid and the differences in their overall intermolecular interaction energies. (C) 1997 American Institute of Physics.