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 Å) 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 ∼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 line 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