Photodissociation of I2-(OCS)n Cluster Ions: Structural Implications

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 2 - access 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 2 - dissociation 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 2 - dissociation on the I - + I( 2 P 3/2 ) asymptote or (3) I 2 - recombination. 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 + I - caging 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 2 - is achieved for n ≥ 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 2 - is comprised of 17 OCS molecules. Results of theoretical calculations of the lowest-energy 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.