Photofragmentation of I2−⋅Ar<i>n</i> clusters: Observation of metastable isomeric ionic fragments

Abstract

We report the 790 nm photofragmentation of mass-selected I−2⋅Arn 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⋅Arm, where m&amp;lt;n), the evaporation process following photoexcitation produces ‘‘solvent-separated’’ (I−...I)⋅Arm photofragments, where the I−2 bond has not reformed. These metastable species comprise ∼55% of the photofragment yield for precursor clusters for n≥14 and have lifetimes ≳5 μs. 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 I−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 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.