Magic sizes of cationic and protonated argon 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 ArnH + 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.

Mixed cationic clusters of nitrogen and hydrogen

“…27,31 Here, the magic numbers for (N 2 ) n D + , most notably n = 2, 7, 12, and 17, match the packing of N 2 molecules around a (N 2 -D-N 2 ) + ionic core in icosahedral (sub-)shells, like was seen for the rare gases. 31,32 Calculated structures of (N 2 ) n H + clusters are shown in Figure 3 where the packing of increases. Our calculations indicate that the octahedral structure of (N 2 ) 6 H + is slightly favorable over (N 2 ) 7 H + with its pentagonal bipyrimid structure, which is consistent with the pair of magic numbers at these sizes found in the experimental data and their relative abundances.…”

“…like as is seen for cationic rare gas clusters. 31 The structure of this cluster is similar to the (N 2 ) 19 H + with an additional pair of N 2 molecules forming end-caps along the central axis.…”

“…The appearance of multiple distinct magic numbers for the protonated clusters mimics results with protonated rare gas clusters. 31,32 In the case of rare gases, the presence of a proton has a stabilizing effect on the clusters compared to pure, cationic systems. This leads to magic numbers for the protonated systems that match predictions for neutral rare gases and sphere packing models.…”

“…Again, this matches the trends observed for protonated rare gas clusters. 31,32 Increasing the relative yield of deuterium in our apparatus, we are able produce clusters mixes of N 2 and intact D 2 . Figure 4 shows a selection of extracted ion yields as a function of the number of N 2 molecules for systems containing 1, 3, 5, and 53 D atoms.…”

“…This method for producing and analyzing weakly bound clusters has been used on numerous occasions in the past and shown to give results that are directly comparable to other methods for cluster production, such as supersonic expansion. 31…”