On the Size and Structure of Helium Snowballs Formed around Charged Atoms and Clusters of Noble Gases

Abstract

Helium nanodroplets doped with argon, krypton, or xenon are ionized by electrons and analyzed in a mass spectrometer. HenNgx+ ions containing up to seven noble gas (Ng) atoms and dozens of helium atoms are identified; the high resolution of the mass spectrometer combined with advanced data analysis make it possible to unscramble contributions from isotopologues that have the same nominal mass but different numbers of helium or Ng atoms, such as the magic He2084Kr2+ and the isobaric, nonmagic He4184Kr+. Anomalies in these ion abundances reveal particularly stable ions; several intriguing patterns emerge. Perhaps most astounding are the results for HenAr+, which show evidence for three distinct, solid-like solvation shells containing 12, 20, and 12 helium atoms. This observation runs counter to the common notion that only the first solvation shell is solid-like but agrees with calculations by Galli et al. for HenNa+ [21568337J. Phys. Chem. A20111157300] that reveal three shells of icosahedral symmetry. HenArx+ (2 ≤ x ≤ 7) ions appear to be especially stable if they contain a total of n + x = 19 atoms. A sequence of anomalies in the abundance distribution of HenKrx+ suggests that rings of six helium atoms are inserted into the solvation shell each time a krypton atom is added to the ionic core, from Kr+ to Kr3+. Previously reported strong anomalies at He12Kr2+ and He12Kr3+ [KimJ. H.16774401J. Chem. Phys.2006124214301] are attributed to a contamination. Only minor local anomalies appear in the distributions of HenXex+ (x ≤ 3). The distributions of HenKr+ and HenXe+ show strikingly similar, broad features that are absent from the distribution of HenAr+; differences are tentatively ascribed to the very different fragmentation dynamics of these ions.

Adsorption of sodium and cesium on aggregates of C60

“…For example, the abundance distribution of ArHe + n which reveals three anomalies [50] is in striking agreement with computed dissociation energies [51]. Unfortunately, the only system for which energies of (C 60 ) m Cs z+ n have been computed versus a continuous range of n values, thus allowing the extraction of D n values, is C 60 Li z+ n with z = 0, 1, 2, and n 14 [3].…”

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen

“…Bartl et al [25] have recently reported equivalent shells for He n Ar þ and draw an analogy between icosahedral He 12 Ar þ and C þ 60 , where the coating of the latter by helium leads to a rigid helium layer [28]. It is the corrugations on the C þ 60 surface that lead to a rigid helium layer, and the same applies to the helium coating of He 12 Ar þ .…”

“…The smallest magic anion is consistent with an icosahedral arrangement of 12 H 2 molecules in the first shell around a central H − core. The larger magic cluster anions can also be interpreted in terms of icosahedral-like structures by analogy with cationic clusters between atomic ions and helium atoms [25]. Thus, the n ¼ 65 magic anion can be explained by a second shell of 20 H 2 molecules occupying the 20 triangular faces of a regular icosahedron, generating a dodecahedral arrangement.…”

Modelling the mobility of positive ion clusters in normal liquid helium over large pressure ranges

“…The notion of a solid snowball surrounding a positive ion embedded in liquid helium is very well established 11,17,19,22,23 . It has been suggested that increased pressure may cause structural transitions 46 .…”