Dynamics and spectra of a solvated electron in water clusters

The dynamics and spectra of negatively charged water clusters, containing a single excess electron, are investigated. In our calculations the atomic water constituents of the clusters are treated classically while the excess electron is described quantum mechanically using the fast Fourier transform algorithm to solve the Schrödinger equation. Information about ground and excited electronic states corresponding to the equilibrium, finite temperature, ground-state ensemble configurations can be obtained by solving for these states for given nuclear configurations generated via quantum mechanical path-integral molecular dynamics simulations. As an alternative, more efficient way, we introduce the adiabatic simulation method which consists of propagating the nuclei in real time while concurrently annealing the electronic wave functions to their correct values corresponding to the instantaneous, dynamically generated nuclear configurations. The resulting trajectories can be used for analyzing nuclear motion in the ground electronic state as well as for calculating energy distributions for the ground and excited electronic states and the (vertical) excitation line shape. We study the cluster size effect on these quantities, and in particular, by comparing results for(H2O)−64 and (H2O)−128, we conclude that the vertical ionization potential increases while the vertical excitation energy to the bound excited state decreases for larger cluster sizes. For the smallest negatively charged water cluster (H2O)−2, where adiabatic separation of electronic and nuclear motion does not hold, we simulate the time evolution in the TDSCF approximation. The dynamics reveals the close correlation between the electronic binding energy and the cluster dipole, and provides information on intramolecular and intermolecular vibrational motion. Comparison of vibrational density of states evaluated from the nuclear trajectories of the negatively charged and the neutral dimer shows that most of the modes associated with intermolecular motions shift to the red upon electron attachment (a few modes, possibly those associated directly with the magnitude of the total molecular dipole, shift to the blue).