MOLECULAR-DYNAMICS SIMULATIONS OF ION IMPACT ON A SUPPORTED RHODIUM CLUSTER

Small metal clusters on a support are commonly used in heterogeneous catalysis. The outermost layer of these clusters largely determines its catalytic properties. Surface analytic techniques such as secondary ion mass spectrometry (SIMS) and low-energy ion scattering (LEIS or ISS) are used to study the atomic composition and structure. With these techniques the surface is bombarded with low-energy (0.1-10 keV) noble gas ions. The ions deposit part of their kinetic energy and may damage the cluster, thereby influencing the measurements when a subsequent ion impinges on the same cluster. In this paper molecular dynamics simulations are used to study the damage formation in a rhodium cluster containing 147 atoms after the impact of He, Ne and Ar ions of 0.5-5 keV. The simulations show that the metal cluster is very fragile. The rhodium sputter yield is considerably higher than for bulk rhodium and the kinetic energy distribution of the sputtered atoms is strongly affected by the shape of the cluster. It is also shown that for surface analysis low ion doses (7 x 10(12) ions/cm2) are required.