AQUEOUS SOLVATION DYNAMICS WITH A QUANTUM-MECHANICAL SOLUTE - COMPUTER-SIMULATION STUDIES OF THE PHOTOEXCITED HYDRATED ELECTRON

We have used molecular dynamics simulation to explore aqueous solvation dynamics with a realistic quantum mechanical solute, the hydrated electron. The simulations take full account of the quantum charge distribution of the solute coupled to the dielectric and mechanical response of the solvent, providing a molecular-level description of the response of the quantum eigenstates following photoexcitation. The solvent response function Is found to be characterized by a 25 fs Gaussian inertial component (40%) and a 250 fs exponential decay (60%). Despite the high sensitivity of the electronic eigenstates to solvent fluctuations and the enormous fractional Stokes' shift following photoexcitation, the solvent response is found to fall within the linear regime. The relaxation of the quantum energy gap due to solvation is shown to play a direct role in the nonradiative decay dynamics of the excited state electron, as well as in the differing relaxation physics observed between electron photoinjection and transient hole-burning (photoexcitation) experiments. A microscopic examination of the solvation response finds that low frequency translational motions of the solvent play an important role in both the inertial and diffusive portions of the relaxation. Much of the local change in solvation structure is associated with a significant change in size and shape of the electron upon excitation. These results are compared in detail both to previous studies of aqueous solvation dynamics and to ultrafast transient spectroscopic work on the hydrated electron.