Kinetic energy losses of C_(60-2n)~+ (n = 1-5) fragments in sub-keV impact of C_(60)~- on a gold surface

摘要

We have studied the formation of C_(60-2n)~+ (n = 1-5) fragment ions following near-grazing impact of C_(60)~- on a gold surface over the 200-275 eV kinetic energy range. It was found that the most probable values of the kinetic energy distributions of the different fragments do not obey unimolecular kinetic energy loss relations, with respect to the most probable value of the kinetic energy distribution of the observed C_(60)~+. As an explanation, we first consider a situation where the fragment ions are formed by unimolecular fragmentation of scattered C_(60)~+ ions whose kinetic energies are much lower than the most probable value of the observed C_(60)~- distribution, while their vibrational excitation is correspondingly higher. Analysis of the fractional abundances of the different fragment ions, based on an assumed correlation between vibrational excitation and energy loss of the C_(60)~+ parent, reveals that a qualitative agreement with the measured results can be achieved but only under the constraint that above some kinetic energy loss threshold value the vibrational excitation efficiency is nearly unity. We also consider an alternative explanation in which the formation of the fragments is related with a prompt, nonunimolecular fragmentation taking place directly at the surface.