New findings on the sputtering of neutral metal clusters

Neutral copper and aluminum clusters containing up to 20 and 12 atoms, respectively, were observed in the sputtering of the polycrystalline metals by 3.75 or 3.9 keV Ar + ions. The clusters were postionized by 6.4 eV photons from an ArF excimer laser and were mass analyzed in a time-of-flight spectrometer. The yields of the clusters were estimated from their postionized concentrations and are shown for the first time to exhibit a power-law dependence on the number of atoms in the cluster. The kinetic energy distributions of Cu through Cu 6 were measured, and the cluster distributions were found to resemble the atom distribution, in agreement with our earlier copper and aluminum data. Collision-cascade-based models cannot predict the yield and kinetic energy distribution data. Several alternate models are considered, but none is found to be satisfactory for explaining the large cluster emission. For small ( n < 3) clusters, the superposition of two or more mechanisms may help to explain the observations.