Electron Attachment and Electron Ionization of Formic Acid Clusters Embedded in Helium Nanodroplets

We report the results of an experimental study of electron ionization of large helium nanodroplets doped with formic acid (FA). Several homologous series of cluster anions are observed, including [FAn-H]−, undissociated FAn−, and these ions complexed with one or more H2O. Some major features resemble those observed upon sputtering of frozen FA films but they differ significantly from results obtained by electron attachment to bare FA clusters in the gas phase. The FAn− and (H2O)[FAn-H]− series show abrupt onsets above n = 2 and 5, respectively. A prominent resonance in the anion yield occurs at 22.5 eV due to the formation of an intermediate He−*. Also observed are homologous series of [FA-H]− or [FA2-H]− complexed with helium. The cation chemistry is dominated by the production of protonated formic acid clusters, [FAnH]+, but various other homologous cluster ion series are observed as well

Doubly Charged Coronene Clusters – Much Smaller than Previously Observed

The smallest doubly charged coronene cluster ions reported so far, Cor152+, were produced by charge exchange between bare coronene clusters and He2+ [H. A. B. Johansson et al., Phys. Rev. A 84, 043201 (2011)]. These dications are at least five times larger than the estimated Rayleigh limit, i.e., the size at which the activation barrier for charge separation vanishes. Such a large discrepancy is unheard of for doubly charged atomic or molecular clusters. Here we report the mass spectrometric observation of doubly charged coronene trimers, produced by electron ionization of helium nanodroplets doped with coronene. The observation implies that Cor32+ features a non-zero fission barrier too large to overcome under the present experimental conditions. The height of the barriers for the dimer and trimer has been estimated by means of density functional theory calculations. A sizeable barrier for the trimer has been revealed in agreement with the experimental findings

Complexes with Atomic Gold Ions:Efficient Bis-Ligand Formation

Complexes of atomic gold with a variety of ligands have been formed by passing helium nanodroplets (HNDs) through two pickup cells containing gold vapor and the vapor of another dopant, namely a rare gas, a diatomic molecule (H2, N2, O2, I2, P2), or various polyatomic molecules (H2O, CO2, SF6, C6H6, adamantane, imidazole, dicyclopentadiene, and fullerene). The doped HNDs were irradiated by electrons; ensuing cations were identified in a high-resolution mass spectrometer. Anions were detected for benzene, dicyclopentadiene, and fullerene. For most ligands L, the abundance distribution of AuLn+ versus size n displays a remarkable enhancement at n = 2. The propensity towards bis-ligand formation is attributed to the formation of covalent bonds in Au+L2 which adopt a dumbbell structure, L-Au+-L, as previously found for L = Xe and C60. Another interesting observation is the effect of gold on the degree of ionization-induced intramolecular fragmentation. For most systems gold enhances the fragmentation, i.e., intramolecular fragmentation in AuLn+ is larger than in pure Ln+. Hydrogen, on the other hand, behaves differently, as intramolecular fragmentation in Au(H2)n+ is weaker than in pure (H2)n+ by an order of magnitude

Electron Attachment and Electron Ionization of Helium Droplets Containing Clusters of C60 and Formic Acid

High-resolution mass spectra of helium droplets doped with C60 and formic acid (FA) are ionized by electrons. Positive ion mass spectra reveal cluster ions [(C60)pFAn]+ together with their hydrogenated and dehydrogenated counterparts. Also observed are ions containing one or more water (W) molecules. The abundance distributions of these ions reveal several interesting features: i) [(C60)pFAn]+ ions are more abundant than hydrogenated [(C60)pFAnH]+ ions even though the opposite is true in the absence of C60 (i.e. if p = 0); ii) although [C60FA]+ is the most abundant ion containing a single C60, multiple C60 suppress the [(C60)pFA]+ signal; iii) an enhanced stability of [(C60)pW1FA5H]+ and [(C60)pW2FA6H]+ mirrors that of [W1FA5H]+ and [W2FA6H]+, respectively. On the other hand, the enhanced stability of [C60FA6H]+ finds no parallel in the stability pattern of [FAnH]+ or FAn+. Negative ion mass spectra indicate a propensity for non-dissociated [(C60)pFAn]- anions if p ≥ 1 which contrasts with the dominance of dehydrogenated [FAn-H]- anions

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms. Under gentle stripping conditions, helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions, some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters of NgnAgm+ at the expense of less stable ones. This results in local anomalies in the cluster ion abundance, which is measured in a high-resolution time-of-flight mass spectrometer. On the basis of these data, we identify specific “magic” sizes n of particularly stable ions. There is no evidence, however, for enhanced stability of Ng2Ag+, in contrast to the high stability of Ng2Au+ that derives from the covalent nature of the bond for heavy noble gases. “Magic” sizes are also identified for Ag2+ dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers n = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for HenH2O+

Electron-induced chemistry in imidazole clusters embedded in helium nanodroplets

Electron-induced chemistry in imidazole (IMI) clusters embedded in helium nanodroplets (with an average size of 2 × 105 He atoms) has been investigated with high-resolution time-of-flight mass spectrometry. The formation of both, negative and positive, ions was monitored as a function of the cluster size n. In both ion spectra a clear series of peaks with IMI cluster sizes up to at least 25 are observed. While the anions are formed by collisions of IMIn with He*–, the cations are formed through ionization of IMIn by He+ as the measured onset for the cation formation is observed at 24.6 eV (ionization energy of He). The most abundant series of anions are dehydrogenated anions IMIn–1(IMI–H)–, while other anion series are IMI clusters involving CN and C2H4 moieties. The formation of cations is dominated by the protonated cluster ions IMInH+, while the intensity of parent cluster cations IMIn+ is also observed preferentially for the small cluster size n. The observation of series of cluster cations [IMInCH3]+ suggests either CH3+ cation to be solvated by n neutral IMI molecules, or the electron-induced chemistry has led to the formation of protonated methyl-imidazole solvated by (n – 1) neutral IMI molecules