Fragmentation and ion-scattering in the low-energy collisions of small silver cluster ions (Agn+: n=1−4) with a highly oriented pyrolytic graphite surface

Abstract

Fragmentation and ion-scattering processes in the low-energy (0–200 eV) collisions of Agn+ (n=1–4) with a highly oriented pyrolytic graphite (HOPG) surface have been investigated by employing a tandem time-of-flight mass spectrometer. It was found that the fragmentation of scattered cluster ions is due to unimolecular dissociation in the energy range studied. A marked difference between the fragmentation pattern of incident Ag3+ and that of Ag4+ has been found: The intact scattered cluster ion was observed for Ag3+ while only fragment ions for Ag4+. From the incident energy dependence of fragment ion intensities, it was deduced that internal energies of the scattered parent ions have upper and lower limits. These limitations are probably due to the adsorption and the implantation of the projectile clusters on the surface. Ion-scattering yield was found to increase with cluster size. Both the incident energy and cluster size dependencies of ion-scattering yields were reproduced by a model calculation in which the hole survival and sticking probabilities of the clusters, and the incident energy spread of the projectile ions are incorporated. The hole survival probability was evaluated from the time-dependent electron transfer rate for a elastically scattered sphere with an equivalent volume to that of the incident cluster ion. The calculation indicates that the average hole–surface distance at the moment of impact, which is determined by the dimensions of the incident cluster, is important for the hole survival in the cluster–surface collisions.