CONTROLLING NONPOLAR SOLVATION TIME SCALES - AN INSTANTANEOUS NORMAL-MODE VIEWPOINT

The effects of temperature, solvent mass, ground-state solute-solvent interaction potential, and difference potential on the time scale for the decay of an electronic transition energy gap correlation function (ECF) are investigated within the context of a linear instantaneous no rmal mode (INM) model of fluid dynamics. This correlation function is also known as the solvation autocorrelation. The system described here is the B-state transition of methyl iodide in the nonpolar solvents a rgon and methane. The required ground-and excited stale interaction po tentials have been determined in previous experimental spectroscopic s tudies. The solvation time scale is of the order of 100-200 fs for sol vent densities ranging from rho = 0.08 to rho* = 0.8. The molecular p roperties responsible for determining the solvation time scale of this non-polar system are delineated here. Via this INM approach, the nonp olar solvation lime scale can be approximated by the ratio of a charac teristic solute-solvent separation distance scaled by the shape of the difference potential and the inertial velocity of the solvent particl es. The time scale of solvation is found to be independent of the magn itude of the difference potential (solute-solvent coupling strength). Thus by changing the coupling strength and leaving the shape of the di fference potential constant, the corresponding electronic absorption s pectrum passes from the inhomogeneous to the motional narrowing limit. This is due to the change in the decay time of the static dipole corr elation function and not to any change in system dynamics. Only very m odest changes in this decay time are found for realistic temperature i ncreases and mass changes of the solvent. Similarly, changes in the gr ound-state solute-solvent potential are found to have only a minimal e ffect on the ECF decay time. Finally, if the shape of the difference p otential is similar for two different observables in a given solvent, the use of the spectral density of one for the description of the (ult rafast) solvent response of the other observable is rationalized. (C) 1997 American Institute of Physics. [S0021-9606(97)51847-2].