Collisional dissociation induced by impact of a cluster anion, ICl-(CO2)n(n=0-20), onto a silicon surface was studied by measuring the branching fractions of the ICl- dissociation and the translational energies of the product anions as functions of n and the collision energy (per ICl-) of ICl-(CO2)n in an apparatus based on a tandem time-of-flight mass spectrometer. It was found that the branching fraction of the ICl- dissociation did not change with n at the collision energies of 30-70 eV. Molecular dynamics simulation showed that the impinging core ion, ICl-, tends to orient with the molecular axis of the core ion being at the angle of 55° with respect to the surface normal, whereas in the X- 2(CO2)n(X=Br, I) collision, the molecular axis of the X- 2 core ion being in parallel to the surface plane. This finding together with prominent wedge effect in the X- 2 (CO2)n collision leads us to conclude that the CO2 molecules do not work as a "wedge" in the ICl-(CO2)n collision because of the unfavorable orientation of the incoming cluster anion. No discernible cage effect could also be related to the orientation. The translational energies of the product anions were interpreted in terms of energy redistribution of the collision energy among the degrees of freedom of the cluster anion and the surface atoms involved in the collision. It was also found that the I and Cl product ions reach quasiequilibrium with the surface by measuring these translational energy distributions.
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