Simulation of cluster ejection following high - energy Au n Au ( 111 ) bombardment

Following the recent report by Andersen et al. of extremely large nonlinear sputtering yields caused by the bombardment of gold targets with small, high energy gold clusters [Phys. Rev. Lett. 80 (1998) 5433], we carried out simulations showing: (1) that the thermal spike phase of the collision cascade contributes significantly to these nonlinear yields; and (2) that individual 'mega-events' with yields up to ~2500 contribute significantly to the total sputtering yields [Shapiro and Tombrello, Nucl. Instrum. Meth. B 152 (1999) 221]. Here we extend our simulations to examine the cluster component of the sputtering yield. We find that clusters with n≥2 constitute a large fraction of the yield. In the early stages of sputtering (≤3 ps) clusters with as many as ~800 atoms are ejected. These very large clusters are not stable; however, after allowing the simulations to evolve for an additional 100 ps, clusters with as many as ~300 atoms remain. The yields of clusters with n≤~10 follow typical power law distributions, while the yield of clusters with n>~10 significantly exceed the predictions of the power-law distribution. Our simulations also show that clusters with more than a few tens of atoms are emitted exclusively during the thermal spike phase. 1. Introduction (Y) was ~3000 for Au 5 clusters, but only ~55 for monomers with the same energy. In an attempt to understand the origin of these large nonlinearit-Andersen et al. [1] reported yield measurements ies, we carried out molecular dynamics (MD) for Au n Au sputtering with 1≤n≤5 at bombard-simulations on a massively parallel computer [2]. ing energies ranging from ~20 keV atom−1 to These simulations showed: ~2 MeV atom−1 for n≤3, and at bombarding 1. that the thermal spike phase of the collision energies ranging from ~20 to ~500 keV atom−1 cascade contributed significantly to the nonlin-for n=4 and n=5. Very large nonlinearities were ear yield, and observed in these sputtering yields. For example, 2. that individual sputtering events with very large at 150 keV atom−1 bombarding energy the yield yields (up to ~2500 atoms) contributed significantly to the total sputtering yield. The emission of clusters is known to be correlated