An instantaneous normal mode analysis of solvation: Methyl iodide in high pressure gases

An instantaneous normal mode (INM) analysis of the short-time solvation dynamics of the B-state (200 nm) Rydberg excitation of methyl iodide in high pressures of Ar (ρ*=0.08, 0.3, and 0.8) is presented. Solute-solvent interaction potentials for this system have been determined by previous absorption and resonance scattering studies. The B-state transition energy correlation function (ECF), also known as the solvation correlation function, calculated by the linear coupling INM theory is in good agreement with the ECF given by molecular dynamics simulation at short times (≤150 fs) that are well beyond the so-called inertial regime (≤100 fs). The shape and peak frequency of the solvation spectra are relatively constant over the wide range of bath densities considered here in contrast to the INM total density of states. This is attributed to the relative density independence of the first peak in the solute-solvent pair distribution function. Similarly, the ECFs are also only modestly dependent on solvent density. A cancellation of the density dependence of the solvation spectrum area and the second moment of the absorption spectrum line shape, and the nearly constant solvation spectrum shape, accounts for the relatively weak density dependence of the ECF decay. A computationally fast, semianalytical method for calculating the weighted density of states incorporating both two- and three-body correlations is shown to be in reasonable agreement with the total INM weighted density of states. A participation ratio analysis of the eigenvectors contributing to the solvation spectrum reveals that single solvent-solute interactions are responsible for the solvation response of the ρ*=0.08 and 0.3 solutions. More collective, totally symmetric solvent motions involving just a few solvent particles, in addition to single solvent interactions, contribute to the solvation response at the liquidlike density of ρ*=0.8. The effects of solvent-solvent repulsions on the shape of the solvation spectrum at this density are also evident by this INM analysis and, in part, account for the modest increase in ECF decay rate at the highest density studied here.