No abstract

Helium droplets provide the possibility to study phenomena at the very low temperatures at which quantum mechanical effects are more pronounced and fewer quantum states have significant occupation probabilities. Understanding the migration of either positive or negative charges in liquid helium is essential to comprehend charge-induced processes in molecular systems embedded in helium droplets. Here, we report the resonant formation of excited metastable atomic and molecular helium anions in superfluid helium droplets upon electron impact. Although the molecular anion is heliophobic and migrates toward the surface of the helium droplet, the excited metastable atomic helium anion is bound within the helium droplet and exhibits high mobility. The atomic anion is shown to be responsible for the formation of molecular dopant anions upon charge transfer and thus, we clarify the nature of the previously unidentified fast exotic negative charge carrier found in bulk liquid helium.

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.

Properties of ground state He(=He(1s 2 1 S)), He + (=He(1s 2 S)), He + 2 (=He 2 (1σ 2 g 1σ u 2 + u )) and excited (metastable) He * (=He(1s2s 3 S)), He 2 * (=He 2 (1σ 2 g 1σ u 2σ g 3 + u )), He * − (=He(1s2s2p 4 P)) and He 2 * − (=He 2 (1σ 2 g 1σ u 2σ g 1π u 4 g )) are calculated using the coupled-cluster method and basis sets multiply augmented with diffuse functions. The aim of this work is to capture the essential physics needed to describe the qualitatively different behaviour of the above mentioned helium species dissolved in liquid helium. By studying their interaction with atomic ground state helium it is found that ground state He, He + , He 2 + and excited (metastable) He * − are well bound within a helium droplet. In comparison excited (metastable) He * , He 2 * and He 2 * − are found to be squeezed out due to the high energetic cost associated with the large volume they require inside a helium droplet. In particular, the molecular species He 2 * and He 2 * − consist of a positive core in the form of a He 2 + which is surrounded by a diffuse electronic cloud accounting for one or two electrons, respectively. The implications of these results for recent experimental studies on helium nanodroplets are discussed, particularly for the negatively charged species He * − and He 2 * − . We find that the latter species experience completely different dynamcis in a helium droplet although they are very similar in various other respects (e.g. diffuse electron clouds, size) in good agreement with experimental observations.

A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu The Journal of Chemical Physics 132, 154104 (2010) Helium droplets are doped with fullerenes (either C 60 or C 70 ) and hydrogen (H 2 or D 2 ) and investigated by high-resolution mass spectrometry. In addition to pure helium and hydrogen cluster ions, hydrogen-fullerene complexes are observed upon electron ionization. The composition of the main ion series is (H 2 ) n HC m + where m = 60 or 70. Another series of even-numbered ions, (H 2 ) n C m + , is slightly weaker in stark contrast to pure hydrogen cluster ions for which the even-numbered series (H 2 ) n + is barely detectable. The ion series (H 2 ) n HC m + and (H 2 ) n C m + exhibit abrupt drops in ion abundance at n = 32 for C 60 and 37 for C 70 , indicating formation of an energetically favorable commensurate phase, with each face of the fullerene ion being covered by one adsorbate molecule. However, the first solvation layer is not complete until a total of 49 H 2 are adsorbed on C 60 + ; the corresponding value for C 70 + is 51. Surprisingly, these values do not exhibit a hydrogen-deuterium isotope effect even though the isotope effect for H 2 /D 2 adsorbates on graphite exceeds 6%. We also observe doubly charged fullerene-deuterium clusters; they, too, exhibit abrupt drops in ion abundance at n = 32 and 37 for C 60 and C 70 , respectively. The findings imply that the charge is localized on the fullerene, stabilizing the system against charge separation. Density functional calculations for C 60 -hydrogen complexes with up to five hydrogen atoms provide insight into the experimental findings and the structure of the ions. The binding energy of physisorbed H 2 is 57 meV for H 2 C 60 + and (H 2 ) 2 C 60 + , and slightly above 70 meV for H 2 HC 60 + and (H 2 ) 2 HC 60 + . The lone hydrogen in the odd-numbered complexes is covalently bound atop a carbon atom but a large barrier of 1.69 eV impedes chemisorption of the H 2 molecules. Calculations for neutral and doubly charged complexes are presented as well.

Dissociative electron attachment to dialanine and alanine anhydride has been studied in the gas phase utilizing a double focusing two sector field mass spectrometer. We show that low-energy electrons (i.e., electrons with kinetic energies from near zero up to 13 eV) attach to these molecules and subsequently dissociate to form a number of anionic fragments. Anion efficiency curves are recorded for the most abundant anions by measuring the ion yield as a function of the incident electron energy. The present experiments show that as for single amino acids (M), e.g., glycine, alanine, valine, and proline, the dehydrogenated closed shell anion (M-H) − is the most dominant reaction product. The interpretation of the experiments is aided by quantum chemical calculations based on density functional theory, by which the electrostatic potential and molecular orbitals are calculated and the initial electron attachment process prior to dissociation is investigated.

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

Dissociative electron attachment to gas phase glycine generates a number of fragment ions, among them ions observed at the mass numbers 15, 16 and 26 amu. From stoichiometry they can be assigned to the chemically rather different species NH(-)/CH(3)(-)(15 amu), O(-)/NH(2)(-)(16 amu) and CN(-)/C(2)H(2)(-)(26 amu). Here we use a high resolution double focusing two sector mass spectrometer to separate these isobaric ions. It is thereby possible to unravel the decomposition reactions of the different transient negative ions formed upon resonant electron attachment to neutral glycine in the energy range 0-15 eV. We find that within the isobaric ion pairs, the individual components generally arise from resonances located at substantial different energies. The corresponding unimolecular decompositions involve complex reaction sequences including multiple bond cleavages and substantial rearrangement in the precursor ion. To support the interpretation and assignments we also use (13)C labelling of glycine at the carboxylic group.