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

Small metal clusters on a support are commonly used in heterogeneous c atalysis. The outermost layer of these clusters largely determines its catalytic properties. Surface analytic techniques such as secondary i on mass spectrometry (SIMS) and low-energy ion scattering (LEIS or ISS ) are used to study the atomic composition and structure. With these t echniques 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 i on impinges on the same cluster. In this paper molecular dynamics simu lations are used to study the damage formation in a rhodium cluster co ntaining 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 rho dium sputter yield is considerably higher than for bulk rhodium and th e kinetic energy distribution of the sputtered atoms is strongly affec ted by the shape of the cluster. It is also shown that for surface ana lysis low ion doses (7 x 10(12) ions/cm2) are required.