PHOTODISSOCIATION OF I-2(-)(OCS)(N) CLUSTER IONS - STRUCTURAL IMPLICATIONS

We report product distributions from the photodissociation of I-2(-)(O CS)(n) (n = 1-26) cluster ions at 790 and 395 nm and discuss implicati ons concerning the structure of these clusters. The experimental resul ts are paralleled by a theoretical investigation of I-2(-)(OCS)(n) str uctures. The 790 and 395 nm transitions in I-2(-) access dissociative excited states that correlate with the I- + I(P-2(3/2)) and I- + I(P- 2(1/2)) products, respectively. Photoabsorption by I-2(-)(OCS)(n) clus ters 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 d istinct pathways: (1) I-2(-) dissociation on the I- + I(P-2(1/2)) spi n-orbit excited asymptote, competing with the solvent-induced spin-orb it relaxation of I(P-2(1/2)) followed by either (2) I-2(-) dissociati on on the I- + I(P-2(3/2)) asymptote or (3) I-2(-) recombination. Path ways 1 and 2 result in a bimodal distribution of the uncaged I-(OCS)(k ) fragments that energetically correlate with the two spin-orbit state s of the escaping I atom. The I + I- caging efficiency is determined a s 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 greater than or equal to 17. For 395 nm excitation, addition of the 17th OCS molecule to I-2(-)(OCS)(16) results in a steplike increase i n 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 CO2 in defining the I-2(-)( OCS)(n) and I-2(-)(CO2)(n) cluster structures are discussed, based on comparison of the photofragment distributions.