We report on the production and study of stable, highly charged droplets of superfluid helium. Using a novel experimental setup we produce neutral beams of liquid helium nanodroplets containing millions of atoms or more that can be ionized by electron impact, mass-per-charge selected, and ionized a second time before being analyzed. Droplets containing up to 55 net positive charges are identified and the appearance sizes of multiply charge droplets are determined as a function of charge state. We show that the droplets are stable on the millisecond time scale of the experiment and decay through the loss of small charged clusters, not through symmetric Coulomb explosions.

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.

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C is added. The sole exception is atomic Cs, which remains at the surface.

High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal He n Au+ cluster ions. Additional doping with heavy noble gases results in Ne n Au+, Ar n Au+, Kr n Au+, and Xe n Au+ cluster ions. The high stability predicted for covalently bonded Ar2Au+, Kr2Au+, and Xe2Au+ is reflected in their relatively high abundance. Surprisingly, the abundance of Ne2Au+, which is predicted to have zero covalent bonding character and no enhanced stability, features a local maximum, too. The predicted size and structure of complete solvation shells surrounding ions with essentially nondirectional bonding depends primarily on the ratio σ* of the ion–ligand versus the ligand–ligand distance. For Au+ solvated in helium and neon, the ratio σ* is slightly below 1, favoring icosahedral packing in agreement with a maximum observed in the corresponding abundance distributions at n = 12. He n Au+ appears to adopt two additional solvation shells of I h symmetry, containing 20 and 12 atoms, respectively. For Ar n Au+, with σ* ≈ 0.67, one would expect a solvation shell of octahedral symmetry, in agreement with an enhanced ion abundance at n = 6. Another anomaly in the ion abundance at Ar9Au+ matches a local maximum in its computed dissociation energy.

Helium nanodroplets are doped with cesium and molecular hydrogen and subsequently ionized by electrons. Mass spectra reveal HxCs + ions that contain as many as 130 hydrogen atoms. Two features in the spectra are striking: First, the abundance of ions with an odd number of hydrogen atoms is very low; the abundance of HCs + is only 1 % that of H2Cs + . The dominance of even-numbered species is in stark contrast to previous studies of pure or doped hydrogen cluster ions. Second, the abundance of (H2)nCs + features anomalies at n = 8, 12, 32, 44, and 52. Guided by previous work on ions solvated in hydrogen and helium we assign the anomalies at n = 12, 32, 44 to the formation of three concentric, solid-like solvation shells of icosahedral symmetry around Cs + . Preliminary density functional theory calculations for n  14 are reported as well.

Features of the solvation of Cs+ by H2 or D2 are unveiled by means of a combined experimental and computational work.

Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some experiments have shown promising results. We investigate the upper bounds of hydrogen storage capacities of C60Cs clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. On bare C60 + , a commensurate phase with 32 H2 molecules was identified in previous experiments. Doping C60 with a single cesium atom leads to an increase in relative ion abundance for the first 10 H2 molecules, and the closure of the commensurate phase is shifted from 32 to 42 H2 molecules.Density functional theory calculations indicate that thirteen energetically enhanced adsorption sites exist, where six of them fill the groove between Cs and C60 and 7 are located at the cesium atom. We emphasize the large effect of the quantum nature of the hydrogen molecule on the adsorption energies, i.e. the adsorption energies are decreased by around 50% for (H2)C60Cs and up to 80% for (H2)C60 by harmonic zero-point corrections, which represent an upper bound to corrections for dissociation energies (De to D0) by the vibrational ground states. Five normal modes of libration and vibration of H2 physisorbed on the substrate contribute primarily to this large decrease in adsorption energies. A similar effect can be found for H2 physisorbed on benzene and is expected to be found for any other weakly H2-binding substrate. © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http:// creativecommons.org/licenses/by-nc-nd/4.0/

Adducts formed between small gold cluster cations and helium atoms are reported for the first time. These binary ions, Aun+Hem, were produced by electron ionization of helium nanodroplets doped with neutral gold clusters and were detected using mass spectrometry. For a given value of n, the distribution of ions as a function of the number of added helium atoms, m, has been recorded. Peaks with anomalously high intensities, corresponding to so-called magic number ions, are identified and interpreted in terms of the geometric structures of the underlying Aun+ ions. These features can be accounted for by planar structures for Aun+ ions with n ≤ 7, with the addition of helium having no significant effect on the structures of the underlying gold cluster ions. According to ion mobility studies and some theoretical predictions, a 3-D structure is expected for Au8+. However, the findings for Au8+ in this work are more consistent with a planar structure.