The formation of clusters during ion induced sputtering of metals

The formation of neutral clusters during sputtering of polycrystalline Ag, Al, Nb and Ta surfaces was studied experimentally by non-resonant single photon post-ionization using a UW or VUV laser beam and time-of-flight mass spectrometry. From the laser intensity dependence of the resulting mass spectra, photoionization cross-sections as well as the yields of sputtered clusters are determined as a function of the cluster size. It is shown that in all cases investigated so far the cluster yields roughly exhibit a power law dependence on the cluster size, the exponent of which is found to be inversely correlated with the sputtering yield of the sample. This finding is of particular importance, since it rules out simple statistical combination models to describe the formation of large sputtered clusters. From the yield distributions it is inferred that depending on the sputtering conditions - up to 46% of the sputtered atoms may be emitted in a bound state. The experimental results are briefly compared with theoretical model descriptions of the cluster emission/formation process available in the literature, It is found that only Molecular Dynamics computer simulations using modern many body potentials are able to reproduce the experimental findings.