Arn clusters: Observation of metastable isomeric ionic fragments

We report the 790 nm photofragmentation of mass-selected I2 2 •Arn clusters, n51 to 27. We determine the I1I caging efficiency as a function of the number of solvent Ar atoms and compare these results with I2 2 in CO2 clusters. Caging is much less effective with Ar. In addition to ‘‘normal’’ caged photoproducts ~I2 2 •Arm , where m,n!, the evaporation process following photoexcitation produces ‘‘solvent-separated’’ ~I•••I!•Arm photofragments, where the I2 2 bond has not reformed. These metastable species comprise ;55% of the photofragment yield for precursor clusters for n>14 and have lifetimes .5 ms. This unusual photofragment exists either as a trapped excited electronic state or as a solvent-separated pair at an internuclear separation of ;5.5 Å. The photofragmentation data also exhibit the existence of two distinct isomeric forms of the precursor I2 2 •Arn , for n<14. These forms are evaporatively distinct in that one isomer displays highly nonstatistical fragmentation, probably arising from a cluster in which the I2 2 resides on the surface, rather than in the interior. The photofragmentation distribution of the other form exhibits statistical behavior, consistent with the evaporation of an I2 2 solvated inside the cluster. © 1996 American Institute of Physics. @S0021-9606~96!00829-X#