Magic sizes of cationic and protonated argon clusters

Abstract

There has long been a discrepancy between the size distributions of ${\mathrm{Ar}}_{n}{}^{+}$ clusters measured by different groups regarding whether or not magic numbers appear at sizes corresponding to the closure of icosahedral (sub-)shells. We show that the previously observed magic cluster size distributions are likely the result of an unresolved ${\mathrm{Ar}}_{n}{\mathrm{H}}^{+}$ component, i.e., from protonated argon clusters. We find that the proton impurity gives cluster geometries that are much closer to those for neutral rare-gas clusters, which are known to form icosahedral structures, than for the pure cationic clusters, explaining why the mass spectra from protonated argon clusters better matches these structural models. Our results thus show that even small impurities, e.g., a single proton, can significantly influence the properties of clusters.