Unimolecular Decomposition of SputteredAln+,Cun+, andSin+Clusters

The metastable decomposition of sputtered, mass-selected, cluster ions is investigated under ultrahigh-vacuum conditions. Highly excited $\mathrm{Al}_n^{}{}_{}{}^{+}$ and $\mathrm{Cu}_n^{}{}_{}{}^{+}$ cluster ions are found to decompose mainly by evaporation of single neutral atoms, whereas for $\mathrm{Si}_n^{}{}_{}{}^{+}$ clusters, fission is also observed. The decay rates of $\mathrm{Cu}_n^{}{}_{}{}^{+}$ clusters are in agreement with calculations presented by Klots. Some consequences of our results for the interpretation of cluster-ion mass spectra produced by other experimental methods are discussed.