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.
Highlights
Rare-gas clusters are some of the simplest chemical systems studied, with many of their structural properties deduced from basic sphere packing models [1] or classical two-body interactions such as the Lennard-Jones 6-12 potential [2,3]
We show that the previously observed magic cluster size distributions are likely the result of an unresolved ArnH+ 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
Summary
Rare-gas clusters are some of the simplest chemical systems studied, with many of their structural properties deduced from basic sphere packing models [1] or classical two-body interactions such as the Lennard-Jones 6-12 potential [2,3]. In 1981, Echt et al reported that clusters of xenon formed by supersonic expansion and ionized by electron impact showed enhanced abundances at clusters sizes of 13, 19, 23, 25, 55, 71, 81, 87, 101, 135, and 147, which could be explained by sphere packing in the formation of icosahedral structures [1]. This was followed by numerous studies showing similar magic cluster size series in Hen+ [9,10], Arn+ [11,12], Nen+ [13], and Krn+ [14,15] clusters. This conclusion is further motivated by ab initio calculations of pure and protonated argon clusters
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