We simulate and interpret the photodissociation and recombination of I2 - embedded in CO2 clusters using a Hamiltonian that accounts for the strong perturbation of the solute electronic structure by the solvent. The calculated product distributions agree well with the experimental results of Lineberger and co-workers. Excited-state dynamics are more involved than anticipated from the isolated solute potential curves. For example, dissociation does not occur from the A‘ state, and permanent recombination occurs only on the X state, despite the fact that the A state of I2 - is weakly bound. We discuss the role of the cluster environment in bringing about recombination and electronic relaxation in terms of a qualitative model inspired by the theory of electron transfer in solution.

We report product distributions from the photodissociation of I 2 -(OCS) n (n ) 1-26) cluster ions at 790 and 395 nm and discuss implications concerning the structure of these clusters. The experimental results are paralleled by a theoretical investigation of I 2 -(OCS) n structures. The 790 and 395 nm transitions in I 2access dissociative excited states that correlate with the I -+ I( 2 P 3/2 ) and I -+ I*( 2 P 1/2 ) products, respectively. Photoabsorption by I 2 -(OCS) n clusters at 790 nm results in "uncaged" I -(OCS) k and "caged" I 2 -(OCS) k fragments. The 395 nm excitation leads, in general, to three distinct pathways: (1) I 2dissociation on the I -+ I*( 2 P 1/2 ) spin-orbit excited asymptote, competing with the solvent-induced spin-orbit relaxation of I*( 2 P 1/2 ) followed by either (2) I 2dissociation on the I -+ I( 2 P 3/2 ) asymptote or (3) I 2recombination. Pathways 1 and 2 result in a bimodal distribution of the uncaged I -(OCS) k fragments that energetically correlate with the two spin-orbit states of the escaping I atom. The I + Icaging efficiency is determined as a function of the number of solvent OCS molecules at both excitation wavelengths studied. At 790 nm, 100% caging of I 2is achieved for n g 17. For 395 nm excitation, addition of the 17th OCS molecule to I 2 -(OCS) 16 results in a steplike increase in the caging efficiency from 0.25 to 0.68. These results suggest that the first solvent shell around I 2is comprised of 17 OCS molecules. Results of theoretical calculations of the lowestenergy I 2 -(OCS) n cluster structures support this conclusion. The roles of different dominant electrostatic moments of OCS and CO 2 in defining the I 2 -(OCS) n and I 2 -(CO 2 ) n cluster structures are discussed, based on comparison of the photofragment distributions.

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