The isotope effect in solvation dynamics and nonadiabatic relaxation: A quantum simulation study of the photoexcited solvated electron in D2O

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Abstract
Quantum nonadiabatic molecular dynamics simulations are used to explore the molecular details surrounding photoexcitation of solvated electrons in deuterated water. The results are compared to previous studies in normal water [B. J. Schwartz and P. J. Rossky, J. Chem. Phys. 101, 6902, 6917 (1994)] to elucidate the nature of the isotope effect on both the solvation and nonadiabatic relaxation dynamics. The solvent spectral density couples differently to the individual energy levels than to the quantum energy gap, indicating the importance of the symmetry of both the ground and excited states in determining the resulting solvent response. The solvation dynamics are characterized by a Gaussian plus biexponential decay. Deuteration has little effect on the Gaussian component or long time exponential decay of the solvent response function, but a ∼20% isotope effect is observed on the faster exponential decay. The solvent response following nonadiabatic relaxation is found to be much more rapid than that following photoexcitation, reflecting the importance of short range mechanical forces and molecular shape in solvation dynamics. Simulated spectral dynamics of the individual ground state bleach, excited state absorption, and stimulated emission components in deuterated water are presented and the results compared to those in normal water. The spectral isotope dependence results principally from the difference in calculated nonadiabatic relaxation rates, which are a factor of ∼2 slower in D2O than H2O. Using the fact that a separate analysis of the quantum decoherence times for the electron suggests that the nonadiabatic transition rates in the two solvents should be identical, calculated spectral transients are corrected for the case of identical nonadiabatic lifetimes and show essentially identical behavior in light and heavy water, in agreement with current experimental results. The small isotope effect on the solvation response should be observable with higher time resolution.