ENERGY AND YIELD DISTRIBUTIONS OF CALCIUM ATOMS AND CLUSTERS UNDERGOING 4 KEV AR-ION BOMBARDMENT()

To date, studies on neutral clusters ejected from metal surfaces under ion bombardment have been reported for metals having only a limited r ange of dimer dissociation energies (1.6-2 eV). The fact that calcium has a dimer dissociation energy of only 0.14 eV makes it an attractive candidate for expanding our understanding of the role dissociation en ergy plays in cluster formation. In this paper we report the first obs ervations of bare-metal calcium clusters up to the pentamer, as well a s a study of the energy and yield distributions for the atom and clust ers up to the trimer. We report a measured total sputter yield of one calcium atom per normally incident 4 keV Ar+ ion. Results for the ener gy distributions concur with previous findings, whereas yield distribu tion results exhibit remarkably different behavior than expected. The slope of the power law fit to the relative sputter yield distribution (-15) is much steeper than expected, and the relative sputter yield fo r the calcium dimer (0.01) is nearly as high as for other metals studi ed, despite its relatively low dissociation energy. It is suggested th at this high dimer yield can be attributed to a significant fraction o f the sputtered dimers being in bound, excited states. The data also s uggest that the dimer population controls the formation of larger clus ters. (C) 1998 Published by Elsevier Science B.V.