Cluster-surface collision induced dissociation of an I_2^- molecule initially embedded in a I_2^-(CO_2)_n cluster was investigated. Molecular dynamics simulation which provides a microscopic description for energy acquisition in the cluster-surface impact. The trajectory calculations using a realistic Si surface model indicate that the collisions of I_2^-(CO_2)_n with a Si surface can be treated as perfectly elastic ones. The dissociation probability of I_2^- was computed for I_2^-(CO_2)_n with n=0-6 on the basis of a hard-wall model. The size dependence of the dissociation probability ascribable to the wedge effect by a CO_2 molecule located halfway between the I atoms, which is consistent with experimental result by Yasumatsu et al
