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.

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Abstract. We report here experimental results for the electron ionization of large superfluid helium nanodroplets with sizes of about 10 5 atoms that are doped with valine and clusters of valine. Spectra of both cations and anions were monitored with high-resolution time-of-flight mass spectrometry (mass resolution >4000). Clear series of peaks with valine cluster sizes up to at least 40 and spaced by the mass of a valine molecule are visible in both the cation and anion spectra. Ion efficiency curves are presented for selected cations and anions at electron energies up to about 40 eV and these provide insight into the mode of ion formation. The measured onset of 24.59 eV for cations is indicative of valine ionization by He + whereas broad resonances at 2, 10 and 22 eV (and beyond) in the formation of anions speak to the occurrence of various modes of dissociative electron attachment by collisions with electrons or He* − and the influence of droplet size on the relative importance of these processes. Comparisons are also made with gas phase results and these provide insight into a matrix effect within the superfluid helium nanodroplet.

Electron attachment to CO2 embedded in superfluid He droplets leads to ionic complexes of the form (CO2)n– and (CO2)nO– and, at much lower intensities, He containing ions of the form Hem(CO2)nO–. At low energies (<5 eV), predominantly the non-decomposed complexes (CO2)n– are formed via two resonance contributions, similar to electron attachment to pristine CO2 clusters. The significantly different shapes and relative resonance positions, however, indicate particular quenching and mediation processes in CO2@He. A series of further resonances in the energy range up to 67 eV can be assigned to electronic excitation of He and capture of the inelastically scattered electron generating (CO2)n– and two additional processes where an intermediately formed He* leads to the nonstoichiometric anions (CO2)nO–.

We show, both experimentally and theoretically, that the adsorption of CO2 is sensitive to charge on a capturing model carbonaceous surface. In the experiment we doped superfluid helium droplets with C60 and CO2 and exposed them to ionising free electrons. Both positively and negatively charged C60(CO2)n(+/-) cluster ion distributions are observed using a high-resolution mass spectrometer and they show remarkable and reproducible anomalies in intensities that are strongly dependent on the charge. The highest adsorption capacity is seen with C60(+). Complementary density functional theory calculations and molecular dynamics simulations provided insight into the nature of the interaction of charged C60 with CO2 as well as trends in the packing of C60(+) and C60(-). The quadrupole moment of CO2 itself was found to be decisive in determining the charge dependence of the observed adsorption features. Our findings are expected to be applied for the adsorption of CO2 on charged surfaces in general.