Minimum Energy Structures of Br ^-₂ Ar_N Clusters: Implications for Dynamics

Abstract

The dynamics of photexcited dihalide species such as I₂ has been extensively studied (1) in small van der Waals molecules with emphasis placed on the role of intramolecular energy transfer (2). In fluids, the geminate recombination dynamics of I₂ excited to several electronic states has been studied both experimentally by picosecond spectroscopy and theoretically by molecular dynamics (MD) and other techniques (3), In order to bridge the gap between van der Waals molecules and condensed phases it is necessary to study small clusters of between 5 and 100 molecules in molecular beams. Our previous MD simulation study of photoexcitation dynamics of Br₂Ar_N neutrals under beam condition demonstrated a gradual transition between the small molecule limit and the condensed phase limit as the numberof argon atoms was increased from 6 to 70 (4). For clusters of less than about 20 atoms, the Br₂ caging dynamics is extremely sensitive to cluster geometry and loss of argon atoms occurs by impulsive energy transfer from Br₂. For two shell cluster (70 atoms), loss of argon occurs by evaporation after the whole cluster is heated.