Photodissociation of I₂^-(OCS)<i>_n</i> Cluster Ions: Structural Implications

Abstract

We report product distributions from the photodissociation of I2-(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 I2-(OCS)n structures. The 790 and 395 nm transitions in I2- access dissociative excited states that correlate with the I- + I(2P3/2) and I- + I*(2P1/2) products, respectively. Photoabsorption by I2-(OCS)n clusters at 790 nm results in “uncaged” I-(OCS)k and “caged” I2-(OCS)k fragments. The 395 nm excitation leads, in general, to three distinct pathways: (1) I2- dissociation on the I- + I*(2P1/2) spin−orbit excited asymptote, competing with the solvent-induced spin−orbit relaxation of I*(2P1/2) followed by either (2) I2- dissociation on the I- + I(2P3/2) asymptote or (3) I2- 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 I2- is achieved for n ≥ 17. For 395 nm excitation, addition of the 17th OCS molecule to I2-(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 I2- is comprised of 17 OCS molecules. Results of theoretical calculations of the lowest-energy I2-(OCS)n cluster structures support this conclusion. The roles of different dominant electrostatic moments of OCS and CO2 in defining the I2-(OCS)n and I2-(CO2)n cluster structures are discussed, based on comparison of the photofragment distributions.