Dissociation of Br2 - (into Br and Br-) in a cluster anion, Br2 -(CO2)n, by impact of Br2 - (CO2)n on a silicon surface was investigated as a function of the number of CO2 molecules, n, at a collision energy per Br2 - of 30-50 eV. The branching fraction of the Br2 - dissociation used as a propensity of the Br2 - dissociation rate showed a marked n-dependence similar to that observed in the collisional dissociation of 12 -(CO2)n, on a silicon surface. The result is explained in terms of wedge effect in which a CO2 molecule at a midpoint of Br2 - splits the Br2 - bond as a wedge vs. cage effect in which the bond splitting of Br2 - in the CO2 solvent cage is suppressed. The agreement of the n-dependence between the Br2 -(CO2)n and I2 -(CO2)n collisions lends a further support for the validity of the bond splitting by the wedge action. A molecular dynamics simulation reproduces the wedge effect in the Br2 -(CO2)n collision, as well. The cage effect appearing in the vicinity of n=12 corresponds to the complettion of the first CO2 solvent shell as is the case of the I2 -(CO2)n collision.
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