Interrogation off the lattice vibrations of liquids with femtosecond Raman-induced Kerr effect spectroscopy

The issue of local order in molecular liquids, and the influence of such order on molecular dynamics and dielectric relaxations in liquids and solutions, is a topic of substantial importance to theoretical representations of the liquid state and interpretations of solvent effects in solute relaxations and chemical reactivity in solutions. In this report we present the results of new studies of intermolecular interactions and dynamics in the liquid state, which interrogate the intermolecular vibrational modes of the local structure of liquids at constant temperature and pressure. Previously we have utilized time and frequency domain analyses of the optical heterodyne detected optical Kerr effect (OHD OKE) to examine the complicated vibrational and Brownian motions of molecules in pure liquids, and identified the intermolecular origin of the low-frequency Raman band in organic liquids (which is typically centered between 10 and 100 cm^-1). These bands have been attributed to Raman-active restricted orientational motions, or librations. in non-associated molecular liquids, and exhibit widths in the range of 20 to 100 cm^-1. In this paper we address the question of local order in molecular liquids by investigating the intermolecular dynamics and spectra of a series of dilutions of benzene in carbon tetrachloride and methylcyclohexane. Benzene is a simple, nonhydrogen-bonding aromatic liquid, and may be considered representative of this class of liquids