INTERACTION-INDUCED CONTRIBUTIONS TO POLARIZABILITY ANISOTROPY RELAXATION IN POLAR LIQUIDS

We use molecular dynamics simulation to investigate polarizability ani stropy relaxation in two polar liquids, methanol and acetonitrile, whi ch have similar dielectric constants at room temperature, but are very different at the molecular level. Interaction-induced contribution to the polarizability is included using first-order perturbation theory and separated into a component which projects along the sum of molecul ar polarizability anistropies and relaxes through collective reorienta tion and a ''collision induced'' component which relaxes through other mechanisms involving mainly translational motion. We find that intera ction-induced effects on the polarizability anisotropy time correlatio n are important on all relevant time scales, especially for the more p olarizable acetonitrile. In methanol, even though most of the molecula r polarizability is along the CO bond, we find that the OH bond dynami cs make a substantial direct contribution to polarizability anisotropy relaxation. We compare our results to the experimentally determined n uclear portion of the optical Kerr effect response and discuss their i mplications for the use of this response in solvation dynamics theorie s. We find that the short-time optical Kerr response of acetonitrile i s dominated by collision-induced polarizability dynamics, while librat ional orientational dynamics is the main contributor for methanol. (C) 1995 American Institute of Physics.