Femtosecond solvation dynamics in acetonitrile: Observation of the inertial contribution to the solvent response

The solvation dynamics of acetonitrile were characterized by a time resolved fluorescence shift measurement determined via the fluorescence upconversion technique. The solvation response is clearly two part in character. The fast initial relaxation accounts for ∼80% of the amplitude and is well fit by a Gaussian of 120 fs FWHM, giving a decay time of 70 fs. The slower tail is exponential with a decay time of ∼200 fs. Comparison of the results to molecular dynamics simulations performed by Maroncelli [J. Chem. Phys. 94, 2085 (1991)] reveal the fast initial part of the solvent response arises from small amplitude inertial rotational motion of molecules in the first solvation shell. The implications of a large amplitude, rapid inertial Gaussian component in the solvent response for theoretical descriptions of chemical reaction dynamics in solution are discussed.