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

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(P3/2) and I+ I*(P1/2) 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*(P1/2) spin-orbit excited asymptote, competing with the solvent-induced spin-orbit relaxation of I*(P1/2) followed by either (2) I2 dissociation on the I+ I(P3/2) 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 + 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 I2 is achieved for n g 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 lowestenergy 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.