INTERMOLECULAR DYNAMICS OF SUBSTITUTED BENZENE AND CYCLOHEXANE LIQUIDS, STUDIED BY FEMTOSECOND NONLINEAR-OPTICAL POLARIZATION SPECTROSCOPY

Femtosecond time-resolved optical-heterodyne detected Raman-induced Ke rr effect spectroscopy (OHD-RIKES) is shown to be a powerful and compr ehensive tool for studying the intermolecular dynamics occurring in li quids. The observed dynamics include both the underdamped, or coherent inertial motions, and the longer time scale diffusive relaxation. The inertial dynamics include phonon-like intermolecular vibrations, inte rmolecular collisions, and librational caging motions. Data are presen ted and analyzed for a series of five liquids: cyclohexane, methylcycl ohexane, toluene, benzyl alcohol, and benzonitrile, listed in order of increasing polarity. We explore the effects of aromaticity (e.g., met hylcyclohexane vs. toluene), symmetry reduction (cyclohexane vs methyl cyclohexane), and substitution effects (e.g., substituted benzene seri es) on the ultrafast intermolecular dynamics, for a group of molecular liquids of similar size and volume. We analyze the intermolecular dyn amics in both the time and frequency domains by means of Fourier trans formations. When Fourier-transformed into the frequency domain, the OH D-RIKES ultrafast transients of the intermolecular dynamics can be dir ectly compared with the frequency domain spectra obtained from the far -infrared absorption and depolarized Raman techniques. This is done us ing the Gaussian librational caging model of Lynden-Bell and Steele, w hich results in a power-law scaling relation between dipole and polari zability time correlation functions. Last, we use a theoretical treatm ent of Maroncelli and co-workers to model for some of these liquids th e solvation time-correlation function for the solvation of a charge-tr ansfer excited-state chromophore based on the measured neat solvent dy namics.