Splitting a chemical bond with a molecular wedge via cluster-surface collisions

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 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.