Nonadiabatic trajectory studies of NaI(H2O)n photodissociation dynamics

We have investigated the photodissociation dynamics of NaI(H(2)O)(n) [n = 1-4] clusters using the molecular dynamics with quantum transitions method and a quantum mechanics/molecular mechanics description of NaI(H(2)O)(n), which involves a semiempirical valence-bond approach to describe the NaI electronic structure and classical solvent-solvent and solute-solvent interaction potentials. Our simulation results show that the NaI(H(2)O)(n) excited-state population decay upon reaching the NaI curve-crossing region increases with cluster size due to the stabilization of the ionic branch of the NaI excited state by the surrounding water molecules, and the resulting increase in nonadiabatic transition probability. After reaching the curve-crossing region for the first time, however, the excited-state population decay resembles that of bare NaI because of rapid evaporation of 99% and 95% of the water molecules for NaI(H(2)O) and NaI(H(2)O)(n) [n = 2-4], respectively. This extensive evaporation is due to the reversed NaI polarity in the Franck-Condon region of the NaI first excited state, which causes strong repulsive NaI-H(2)O forces and induces rapid nonstatistical water evaporation, where product water molecules are formed more rotationally than translationally hot. A few water molecules (5% or less) remain transiently or permanently bound to NaI, forming long-lived clusters, when NaI remains predominantly ionic, i.e., remains in the excited state, after reaching the curve-crossing region. To connect simulation results with experiment, we have simulated femtosecond probe signals resulting from two-photon and one-photon excitation to the X and I NaI(+) probe states. In agreement with experimental findings, the probe signals resulting from the two-photon probe scheme, where excitation occurs from the covalent branch of the excited state, decay exponentially over the NaI first excited-state vibrational period, with very little evidence of long-time dynamics. The one-photon probe scheme (not used for experimental cluster studies) is shown to be less sensitive to solvation, in that excitation energies will remain similar over a range of cluster sizes, as the ionic branch of the excited state and the NaI(+) probe states are stabilized to the same extent by the presence of water molecules. The resulting probe signals are also more revealing of the NaI(H(2)O)(n) photodissociation dynamics than the two-photon probe signals, as they may allow monitoring of solvation effects on the NaI nonadiabatic dynamics and of successive evaporation of water molecules. Time-resolved photoelectron spectra provide limited additional information regarding the NaI(H(2)O)(n) photodissociation dynamics. A key consequence of the rapid water evaporation demonstrated here is that experimentally observed signals may arise from the photodissociation of much larger NaI(H(2)O)(n) parent clusters.