EJECTION OF MOLECULAR CLUSTERS FROM ION-BOMBARDED SURFACES.

We have modeled, using classical dynamics, the dissipation of momentum of a 600-eV Ar** plus ion as it bombards a metal single crystal. The model correctly predicts relative sputtering yields, secondary particle energy distributions, and angular distributions. In addition, it also gives considerable insight into the mechanism of molecular cluster formation. For the three low index faces of copper, for example, the observed dimers, trimers, and higher multimers form over the surface but within interaction range of the solid. The clusters show rearrangement of their constituent atoms from their original surface positions, but do arise from a localized region of radius similar 5 A. We have also examined oxygen atoms and CO molecules adsorbed on copper and nickel, respectively. For the chemisorbed O atoms, the clusters Cu//2, CuO, O//2, Cu//3, Cu//2O, CuO//2, and O//3 have all been observed to form over the surface, analogous to the clean metal case. For CO, however, most of the ejection occurs molecularly due to the strong carbon-oxygen bond ( similar 11 eV). The formation of NiCO and Ni//2CO clusters occurs over the surface.