Magic Sizes of Cationic and Protonated Argon Clusters

TL;DR

This paper investigates the size distributions of argon clusters and reveals that protonated clusters, rather than pure cationic ones, explain the observed magic numbers and structural properties, highlighting impurity effects.

Contribution

It demonstrates that proton impurities significantly influence argon cluster structures and explains discrepancies in previous experimental observations.

Findings

Protonated argon clusters better match icosahedral models.

Impurities like a single proton can alter cluster properties.

Observed magic numbers are linked to protonated rather than pure clusters.

Abstract

There has long been a discrepancy between the size distributions of 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 Ar$_n$H$^+$ component, that is, 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 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.