Atomically resolved phase transition of fullerene cations solvated in helium droplets

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

Anionic hydrogen cluster ions as a new form of condensed hydrogen

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, Hn- and Dn-, where n >= 5. These anions were formed by electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions were stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of n were detected and some specific ions showed anomalously high abundances. The sizes of these ‘magic number’ ions suggest an icosahedral framework of H2 (D2) molecules in solvent shells around a central H-(D-) ion. The first three shells, which contain a total of 44 H2 or D2 molecules, appear to be solid-like but thereafter a more liquidlike arrangement of the H2 (D2) molecules is adopted

Observation of stable HO4+ and DO4+ ions from ion-molecule reactions in helium nanodroplets

Ion–molecule reactions between clusters of H₂/D₂ and O₂ in liquid helium nanodroplets were initiated by electron-induced ionization (at 70 eV). Reaction products were detected by mass spectrometry and can be explained by a primary reaction channel involving proton transfer from H₃+ or H₃+(H₂)n clusters and their deuterated equivalents. Very little HO₂+ is seen from the reaction of H₃+ with O₂, which is attributed to an efficient secondary reaction between HO₂+ and H₂. On the other hand HO₄+ is the most abundant product from the reaction of H₃+ with oxygen dimer, (O₂)₂. The experimental data suggest that HO₄+ is a particularly stable ion and this is consistent with recent theoretical studies of this ion

Isomeric broadening of C60+ electronic excitation in helium droplets: experiments meet theory

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.(VLID)4795215Version of recor