SIMULATIONS OF CORE EXCITATION IN ENERGETIC CLUSTER IMPACTS ON METALLIC SURFACES

A modified version of the SPUT2 molecular dynamics sputtering code was used to investigate core excitation in Al atoms following the impact of pure Al clusters and composite Al-Au clusters on Au surfaces. Information was obtained on distances-of-closest-approach, times-of-closest-approach, and ejection properties of cluster-atoms undergoing hard collisions for 32-, 63-, and 108-atom Al clusters impacting Au(100) targets with energies/atom ranging from 0.2 keV to 1.0 keV. Similar information was obtained for composite Al-Au clusters (38 Al atoms, 25 Au atoms) with comparable total cluster energies. Using a simple critical distance model (R(c) = 0.44 angstrom) for L-shell core excitation in Al, the threshold for core excitation in pure Al clusters was found to be at approximately 0.4 keV/atom. For the composite clusters, the threshold was at approximately 0.11 keV/Al-atom. Core excitation was most probable during the early, compressional phase of the cluster impacts. A significant fraction (typically approximately 40%) of core-excited Al atoms were found to eject from the cluster-target system. Elapsed times between excitation and ejection typically were less than 80 fs, and significant numbers of Al atoms ejected within 20-30 fs after excitation. Our results suggest that tine spectra of Auger electrons from sputtered. excited cluster-atoms can be used as a diagnostic tool to investigate the early stages of energetic cluster collisions with surfaces.