Short-time dynamics of vibrational relaxation in molecular fluids

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 sufficiently similar to the Lennard-Jones part of the solute-solvent potential that solvation probed by Lennard-Jones potentials ends up sensing a virtually identical mechanism, regardless of the specifics of the liquids. We find that, in both cases, the events that trigger the relaxation typically involve no more than a solvent molecule or two, independently of whether the system is a dipolar solute dissolved in CH3CN or I-2 dissolved in either liquid or supercritical CO2. The similarity even extends to the precise spectra of active instantaneous normal modes of the Liquid that govern the dynamics for the two very different processes (the "influence spectra"). With a much longer ranged probe potential, such as that found in dipolar salvation, the shape of the influence spectrum does become noticeably different, but even for electrostatics-dominated examples we never find that more than four or five solvents are needed to make up the bulk of the influence spectrum derived from any one liquid configuration. The collective character of solute relaxation evidently does not come into play until times significantly longer than the duration of the triggering events.