FEMTOSECOND SOLVATION DYNAMICS OF WATER

THE timescale of the response of solvent molecules to electronic rearrangement of solute molecules has a critical influence on the rates of chemical reactions in liquids(1-10). In particular, if the solvent cannot adapt quickly enough to this rearrangement as the reactants pass through the transition state, the evolving products may recross the free-energy barrier, reducing the reaction rate. Computer simulations have shown(11-18) that the response of a solvent to a change in solute charge distribution is strongly bimodal: there is an initial ultrafast response owing to inertial (mainly librational) motions of tbe solvent molecules, followed by a slow component owing to diffusive motions. Water seems to be by far the (fastest' solvent studied so far: simulations predict that well over half of the solvation response for atomic solutes is inertial, happening on a timescale of about 20 femtoseconds(12,13). The presence of this ultrafast component implies that solvent friction plays an important role in many aqueous charge-transfer processes(9,10,19-21). Experimental verification of this prediction has been lacking, however, in part because of the difficulty of obtaining sufficient time resolution. Here we present experimental measurements of the ultrafast solvation dynamics of a coumarin salt in water. When considered in conjunction with computer simulations, our results demonstrate that a solvent response on a timescale faster than 50 fs can dominate aqueous solvation dynamics.