Recombination and relaxation of molecular ions in size-selected clusters: Monte Carlo and molecular dynamics simulations of I−2 (CO2)n

The equilibrium structures and the recombination dynamics of I−2 molecular ions embedded in clusters of 3–17 CO2 molecules are studied by Monte Carlo and molecular dynamics simulations. The potential model incorporates, in a self-consistent manner, a description of the I−2 electronic structure that depends on both the I−2 bond length and the solvent degrees of freedom. The influence of the solvent upon the I−2 electronic structure is treated by means of a single effective solvent coordinate, in a manner reminiscent of the theory of electron transfer reactions. This causes the electronic charge to localize on a single I atom when the I–I bond is long or when the solvent cage has become highly asymmetric. The primary focus is the I−2 vibrational relaxation that follows recombination. Simulations of I−2(CO2)16 and I−2(CO2)9 yield vibrational relaxation times of less than 3 ps, even faster than the experimentally observed absorption recovery time of 10–40 ps. It is suggested that the latter time scale is determined by electronic as well as vibrational relaxation mechanisms.