Helium has a unique phase diagram and below 25 bar it does not form a solid even at the lowest temperatures. Electrostriction leads to the formation of a solid layer of helium around charged impurities at much lower pressures in liquid and superfluid helium. These so-called ‘Atkins snowballs' have been investigated for several simple ions. Here we form HenC60+ complexes with n exceeding 100 via electron ionization of helium nanodroplets doped with C60. Photofragmentation of these complexes is measured by merging a tunable narrow-bandwidth laser beam with the ions. A switch from red- to blueshift of the absorption frequency of HenC60+ on addition of He atoms at n=32 is associated with a phase transition in the attached helium layer from solid to partly liquid (melting of the Atkins snowball). Elaborate molecular dynamics simulations using a realistic force field and including quantum effects support this interpretation.

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, H − n and D − n , where n ≥ 5. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of n are detected, and some specific ions show anomalously high abundances. The sizes of these "magic number" ions suggest an icosahedral framework of H 2 (D 2 ) molecules in solvent shells around a central H − (D − ) ion. The first three shells, which contain a total of 44 H 2 or D 2 molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the H 2 (D 2 ) molecules is adopted. to be strongly bent rather than linear. Consequently, there are real doubts about the basic structures of H − n ions that need to be resolved, and given that there are no reliable estimates of the actual dissociation energies (D 0 ) of these clusters, it is not even clear if anions with n > 3 are stable.Here we report the first experimental detection of anionic hydrogen and deuterium clusters larger than H − 3 =D − 3 and have done so for a wide range of cluster sizes. The experimental procedure involved the formation of the corresponding neutral ðH 2 Þ N and ðD 2 Þ N clusters by adding H 2 or D 2 gas to liquid helium nanodroplets. The neutral clusters were cooled to the ambient temperature of the helium nanodroplets, 0.38 K [13], prior to an impact by a beam of electrons with a controlled energy. H 2 and D 2 are heliophilic (have a negative chemical potential when immersed in liquid helium [13]) and so will reside inside the helium droplets rather than on the surface. The helium droplets were then exposed to a beam of electrons, which generated anionic products in the gas phase that were detected by mass spectrometry. The transfer of negative charge to the ðH 2 Þ n and ðD 2 Þ N clusters occurs via a mobile electron bubble, whose formation has a threshold energy in excess of 1 eV in order to inject the electron into the helium conduction band [14]. The droplets used in the present work were relatively large (∼10 6 helium atoms), and for droplets of this size the electron bubble, although

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.

The effects of interactions between He − and clusters of fullerenes in helium nanodroplets are described. Electron transfer from He − to (C 60 ) n and (C 70 ) n clusters results in the formation of the corresponding fullerene cluster dianions. This unusual double electron transfer appears to be concerted and is most likely guided by electron correlation between the two very weakly bound outer electrons in He − . We suggest a mechanism which involves long range electron transfer followed by the conversion of He + into He 2 + , where formation of the He-He bond in He 2 + releases sufficient kinetic energy for the cation and the dianion to escape their Coulombic attraction. By analogy with the corresponding dications, the observation of a threshold size of n ≥ 5 for formation of both (C 60 ) n 2− and (C 70 ) n 2− is attributed to Coulomb explosion rather than an energetic constraint. We also find that smaller dianions can be observed if water is added as a co-dopant. Other aspects of He − chemistry that are explored include its role in the formation of multiply charged fullerene cluster cations and the sensitivity of cluster dianion formation on the incident electron energy. C

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.

Electron addition to cobalt tricarbonyl nitrosyl (Co(CO3NO) and its clusters has been explored in helium nanodroplets. Anions were formed by adding electrons with controlled energies, and reaction products were identified by mass spectrometry. Dissociative electron attachment (DEA) to the Co(CO)3NO monomer gave reaction products similar to those reported in earlier gas phase experiments. However, loss of NO was more prevalent than loss of CO, in marked contrast to the gas phase. Since the Co–N bond is significantly stronger than the Co–C bond, this preference for NO loss must be driven by selective reaction dynamics at low temperature. For [Co(CO)3NO]N clusters, the DEA chemistry is similar to that of the monomer, but the anion yields as a function of electron energy show large differences, with the relatively sharp resonances of the monomer being replaced by broad profiles peaking at much higher electron energies. A third experiment involved DEA of Co(CO)3NO on a C60 molecule in an attempt to simulate the effect of a surface. Once again, broad ion yield curves are seen, but CO loss now becomes the most probable reaction channel. The implication of these findings for understanding focused electron beam induced deposition of cobalt is described.

The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He + ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He + and He2 + as a function of electron energy, electron emission current, and droplet size reveals that metastable He *-anions play a crucial role in the formation of free He + at subthreshold energies. The proposed model is testable. Research into helium nanodroplets, originally a scientific niche driven by curiosity about the minimum droplet size that supports superfluidity, 1 has matured to a point where 4 He droplets provide a novel method to synthesize and characterize unusual molecules, large aggregates in unusual morphologies, metallic foam, or nanowires from a wide range of materials.2-7 Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets. 7,8 A topic that has been of interest ever since large helium droplets were efficiently produced in supersonic jets 9 is the mechanism by which droplets become charged by ionizing radiation. How do small Hen + ions containing as few as two atoms emerge from a very large neutral, undoped droplet? 10,11 How do monomer or dimer ions form when the energy of the ionizing radiation is below their thermodynamic threshold? [12][13][14] How do large Hen -cluster anions form upon electron impact? [15][16][17][18] What role do metastable electronically excited species play? 12,19 What is the local structure near a positive or negative charge in undoped helium droplets, and how does it compare to that in bulk helium or helium films? [20][21][22][23] Our present work addresses the formation and subsequent ejection of bare He + from undoped droplets. For ionizing radiation exceeding the ionization threshold of atomic He (24.59 eV) small Hen + cluster ions (n > 1) are thought to result from a two-step mechanism. 12,22,[24][25][26][27][28] The process commences with the formation of He + in the droplet. Direct formation of Hen + cluster ions (n > 1) is disfavored by very small Franck-Condon factors. The hole will hop, on the time scale of femtoseconds, by resonant charge exchange with adjacent helium atoms. After about 10 hops the charge will localize by forming a vibrationally excited He2 + . Its excess energy will be large given the large (about 2.4 eV) dissociation energy of He2 + . 29 This energy would be sufficient to boil off thousands of helium atoms (the bulk cohesive energy of helium is 0.62 meV) but a thermal process appears unlikely; evaporation of even thousands of helium atoms from a primary droplet containing »10 4 helium atoms would still result in a helium cluster ion whose size lies outside the range of...

Helium is considered an almost ideal tagging atom for cold messenger spectroscopy experiments. Although helium is bound very weakly to the ionic molecule of interest, helium tags can lead to shifts and broadenings that we recorded near 963.5 nm in the electronic excitation spectrum of C60+ solvated with up to 100 helium atoms. Dedicated quantum calculations indicate that the inhomogeneous broadening is due to different binding energies of helium to the pentagonal and hexagonal faces of C60+, their dependence on the electronic state, and the numerous isomeric structures that become available for intermediate coverage. Similar isomeric effects can be expected for optical spectra of most larger molecules surrounded by nonabsorbing weakly bound solvent molecules, a situation encountered in many messenger-tagging spectroscopy experiments.