Nonadiabatic molecular dynamics simulation of photoexcitation experiments for the solvated electron in methanol

Abstract

Nonadiabatic quantum molecular dynamics simulations have been performed to simulate the pump-and-probe photoexcitation experiments of the ground state equilibrium solvated electron in methanol carried out by Barbara et al. [Chem. Phys. Lett. 232, 135 (1995)]. We have characterized both the time evolution of the quantum solute, the solvated electron, and the solvation response of the classical methanol bath. The quantum energy gap provides an excellent tool to gain insight into the underlying microscopic details of the solvation process. The solvent response is characterized for both processes by a fast Gaussian component and a biexponential decay. The present results suggest that the residence time of the solvated electron in the first excited state is substantially longer than inferred from the cited experiments. The experimentally observed fast exponential portion of the relaxation more likely corresponds to the adiabatic solvent response than to the lifetime of the excited state electron. By comparing to photoexcitation simulations in water, it is shown that the simulated excited state lifetime is about three times longer in methanol than in water, predicting a less substantial increase than a recent calculation based on nonadiabatic coupling elements alone. Hydrogen-bonding statistical analysis provides interesting additional details about the dynamics. We find that the hydrogen-bonding network is significantly different in the first solvent shell around the electron in ground and first excited states, the distribution around the latter, larger and more diffuse, ion resembling more that of the pure liquid. Transformation of the corresponding hydrogen bonding structures takes place on a 1 ps time scale.