Ultrafast reaction dynamics in cluster ions: Simulation of the transient photoelectron spectrum of I2 Arn photodissociation

The photodissociation and recombination of a dihalide ion embedded in a cluster illustrates the crucial role played by the solvent in chemical reactions that involve charged species. In these strongly interacting systems, the solvent does not merely interrupt and redirect motion on the potential energy surfaces of the isolated molecule, it profoundly modifies the surfaces themselves. As the dihalide solute dissociates in an excited state ~Fig. 1!, the forces exerted on the solute electrons by the solvent molecules become as important as the forces within the solute; both solute and solvent become polarized. This leads to long range forces that can inhibit dissociation in the initially excited state, and to solvent-induced nonadiabatic transitions that can bring about recombination in other electronic states. Finally, the solvent can stabilize the recombined species by dissipating their excess energy. Dihalide anion photodissociation in clusters has been studied extensively both experimentally and theoretically, but several key aspects of the dynamics remain poorly understood. Little is known about the dynamics that take place on the excited state following photoexcitation or about what motions of the solute and solvent are necessary to bring about electronic relaxation and product formation. Full theoretical modeling of these photodissociation reactions is difficult because it requires both a knowledge of the electronic structure of a manifold of states strongly coupled to the many solvent degrees of freedom and a method of computing the complex dynamics on these surfaces. Several groups have recently developed semiempirical Hamiltonians for dihalides and dihalide anions interacting with rare gas atoms. The work of Batista and Coker has demonstrated the important role of nonadiabatic dynamics on the multiple potential surfaces in these systems following photoexcitation. Nonadiabatic molecular dynamics simulations of I2 2 photodissociation in argon clusters from Coker’s group and confirmed by our group have