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 a nisotropic components of the low-frequency, intermolecular Raman spect ra of liquids. The isotropic and anisotropic spectral features are com pared for liquids benzene, toluene, and benzonitrile in order to chara cterize the intermolecular motions in terms of their relative contribu tion to the different elements of the Raman susceptibility. We observe that the spectral profiles for the isotropic and anisotropic componen ts 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 throug hout the 0-150 cm(-1) region, respectively, for benzene and toluene, r espectively. This ratio indicates that the collective intermolecular v ibrations 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 ne ar 0 cm(-1) to approximately 0.5 at 150 cm(-1). This observation indic ates that the intermolecular interactions in benzonitrile become incre asingly ''polarized'' as the frequency is increased. These data are di scussed in terms of the distinct molecular properties of each liquid a nd the differences in their overall intermolecular interaction energie s. (C) 1997 American Institute of Physics.