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

Fragmentation and ion-scattering processes in the low-energy (0-200 eV) col lisions of Ag-n(+) (n = 1-4) with a highly oriented pyrolytic graphite (HOP G) surface have been investigated by employing a tandem time-of-flight mass spectrometer. It was found that the fragmentation of scattered cluster ion s is due to unimolecular dissociation in the energy range studied. A marked difference between the fragmentation pattern of incident Ag-3(+) and that of Ag-4(+) has been found: The intact scattered cluster ion was observed fo r Ag-3(+) while only fragment ions for Ag-4(+). From the incident energy de pendence of fragment ion intensities, it was deduced that internal energies of the scattered parent ions have upper and lower limits. These limitation s are probably due to the adsorption and the implantation of the projectile clusters on the surface. Ion-scattering yield was found to increase with c luster 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 energ y spread of the projectile ions are incorporated. The hole survival probabi lity was evaluated from the time-dependent electron transfer rate for a ela stically scattered sphere with an equivalent volume to that of the incident cluster ion. The calculation indicates that the average hole-surface dista nce at the moment of impact, which is determined by the dimensions of the i ncident cluster, is important for the hole survival in the cluster-surface collisions. (C) 2000 American Institute of Physics. [S0021-9606(00)70333-3] .