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

Abstract

A cluster anion, I2−(CO2)n (n=0−30), was allowed to collide onto a silicon surface at collision energies (per I2−) 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, I2−, dissociates by the collision of I2−(CO2)n on the silicon surface. The branching fraction for the I2− 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 I2−) 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 I2− bond splits the I2− bond as if a piece of wood is split by a hammer thrust against a wedge vs a cage effect, in which the I2− 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 I2− dissociation were also verified by use of the calculation based on a molecular dynamics simulation.