Metallic cluster beams from a 2.5 MV Van de Graaff accelerator for cluster-solid interaction studies:: preliminary results with Aun+ clusters

A liquid metal ion source (LMIS) was installed on the high-voltage terminal of the 2.5 MV single-stage Van de Graaff accelerator of the Institut de Physique Nucleaire de Lyon and mas used to deliver intense MeV energy beams of cluster ions. After acceleration, the ions produced from an eutectic Au-Si alloy were mass-selected by means of a magnetic analysis. The beam was mainly composed of the monoatomic ions Au+ and Si+ and of the clusters Au-n(+), AunSi+, and AunSi2+ (n = 2 - 13). The intensities decreased for increasing n values, but remained remarkably high. For pure gold ions, the maximum intensity measured at the target site varied from 250 nA, for Au+, down to 20 pA, for Au-9(+). The research program on the study of the specific effects of the impact of energetic dusters on solid surfaces concerns the fundamental interaction processes and the solid modifications induced by high-density energy deposits. Our first experiments with Au-n(+) clusters led to the following results: Nonlinear dependence on the cluster size of kinetic secondary electron emission from thin carbon foil enhances the damage rate of irradiated silicon crystals as compared to monoatomic Au+ ion effects.