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 (f(dis)) was determined as functions of the collision energy and the number of the CO2 molecules, n. The marked n-dependence of f(dis) 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 geminate recombination between the dissociating I and I- 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.