Short-Time Dynamics of Vibrational Relaxation in Molecular Fluids

Abstract

Modern approaches to vibrational relaxation in liquids have begun to move beyond the simple question of how fast vibrational population relaxation occurs to the more challenging question of how it occurs at all: the precise molecular mechanisms by which a solvent stimulates the loss of a vibrational quantum from a solute. We report here some progress in understanding these mechanisms based on looking at the dynamics of the initial triggering events in the vibrational relaxation seen in molecular fluids. With the aid of instantaneous-normal-mode analysis we find a remarkable similarity between vibrational relaxation and the dynamics of solvation. The key concept, in both cases, is that the polarity and general behavior of the solvent is far less important in determining the relevant mechanism than is the particular force or potential monitored by the relevant experiment (“the spectroscopic probe potential”). Vibrational population relaxation automatically accesses the force on a bond, a quantity sufficien...