A combined experimental/theoretical investigation of the near-infrared photodissociation of IBr^- (CO₂)_n

We report the collaborative experimental and theoretical study of the time-resolved recombination dynamics of photodissociated IBr^- (CO₂)_n clusters. Excitation of the bare anionic chromophore to the dissociative A′ ²Π_1/2 state yields only I^- and Br products. Interestingly, however, the addition of a few solvent molecules promotes recombination of the dissociating chromophore on the X ²Σ_1/2⁺ ground state, which correlates asymptotically with Br- and I products. This process is studied experimentally using time-resolved, pump-probe techniques and theoretically via nonadiabatic molecular dynamics simulations. In sharp contrast to previous I2- studies where more kinetic energy was released to the photofragments, the observed recombination times increase from picoseconds to nanoseconds with increasing cluster size up to n=10. The recombination times then drop dramatically back to picoseconds for cluster sizes n=11-14. This trend, seen both in experiment and theory, is explained by the presence of a solvent-induced well on the A′ state, the depth of which directly corresponds to the asymmetry of the solvation about the chromophore. The results seen for both the branching ratios and recombination times from experiment and theory show good qualitative agreement.