A cluster anion, I-2 (CO2)n (n=0-30), was allowed to collide onto a silicon surface at collision energies (per I-2) of 1-80 eV in an ultrahigh vacuum surface-collision chamber equipped with a tandem time-of-flight (TOF) mass spectrometer. The product anions show that the core ion, I-2, dissociates by the collision of I-2(CO2)n on the silicon surface. The branching fraction for the I-2 dissociation (fdis) was determined as functions of the collision energy and the number of the CO2 molecules, n. The marked n-dependence of fdis at a collision energy (per I-2) higher than 30 eV was explained in terms of a wedge effect in which a CO2 molecule in the vicinity of the mid point of the I-2 bond splits the I-2 bond as if a piece of wood is split by a hammer thrust against a wedge vs a cage effect, in which the I-2 dissociation is suppressed by gemmate recombination between the dissociating I and A- pair in a complete solvation shell. The wedge and cage effects in the I-2 dissociation were also verified by use of the calculation based on a molecular dynamics simulation.
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