Photofragmentation of I-2(-)center dot Ar-n clusters: Observation of metastable isomeric ionic fragments

We report the 790 nm photofragmentation of mass-selected I-2(-). Ar-n clusters, n=1 to 27. We determine the I-+I caging efficiency as a function of the number of solvent Ar atoms and compare these results with I-2(-) in CO2 clusters. Caging is much less effective with Ar. In addition to ''normal'' caged photoproducts (I-2(-). Ar-m, where m<n), the evaporation process following photoexcitation produces ''solvent-separated'' (I-... I). Ar-m photofragments, where the I-2(-) bond has not reformed. These metastable species comprise similar to 55% of the photofragment yield for precursor clusters for n greater than or equal to 14 and have lifetimes >5 mu s, This unusual photofragment exists either as a trapped excited electronic state or as a solvent-separated pair at an internuclear separation of similar to 5.5 Angstrom. The photofragmentation data also exhibit the existence of two distinct isomeric forms of the precursor I-2(-). Ar-n, for n less than or equal to 14. These forms are evaporatively distinct in that one isomer displays highly nonstatistical fragmentation, probably arising from a cluster in which the I-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 I-2(-) solvated inside the cluster. (C) 1996 American Institute of Physics.