Marcelo Goulart

Electron-driven ionization of large methanol clusters in helium nanodroplets

The electron-driven ionization of helium droplets doped with pure methanol and ethanol clusters has been investigated for the first time using high resolution mass spectrometry. Large clusters are readily accessible by this route, with up to 100 alcohol molecules seen in the present study. The mass spectra for the doped helium droplets show many similarities with previous gas phase mass spectrometric studies of methanol and ethanol clusters. Thus the dominant ion products, at least for small clusters, are the protonated species H(+)(CH(3)OH)(n) and H(+)(C(2)H(5)OH)(n). Likewise intra-cluster reaction is observed to produce H(+)(H(2)O)(CH(3)OH)(n) and H(+)(H(2)O)(C(2)H(5)OH)(n) ions. However, in helium droplets the observation of consecutive intra-cluster reactions is seen with product molecules containing up to five water molecules. The evidence points towards the proton locating on H(2)O to form H(3)O(+), rather than the alcohol, despite the higher proton affinity of the latter. The behaviour of the H(+)(H(2)O)(m)(ROH)(n) ion signals as a function of cluster size is consistent with the most stable cluster structures arising from a central H(3)O(+) ion surrounded by two or more complete five-membered rings with the constituents held in place by hydrogen bonds.

Magic Numbers for Packing Adamantane in Helium Droplets: Cluster Cations, Dications, and Trications

We report the first observation of cations, dications, and trications of large clusters of adamantane. Cluster formation was initiated near 0 K in helium droplets and ionization was achieved with one or more collisions with energetic He species (He*, He+, or He*–). The occurrence of Coulomb explosion appeared to discriminate against the formation of small multiply-charged clusters. High-resolution mass spectrometry revealed the presence of “magic number” m/z peaks that can be attributed to the packing of adamantane molecules into cluster structures of special stability involving preferred arrangements of these molecules. These abundance anomalies were seen to be independent of charge state. While some dehydrogenation of adamantane and its clusters was seen as well, no major transformations into adamantoids or microdiamonds were observed.

Positively and Negatively Charged Cesium and (C60)mCsn Cluster Ions

We report on the formation and ionization of cesium and C60Cs clusters in superfluid helium nanodroplets. Size distributions of positively and negatively charged (C60)mCsn± ions have been measured for m ≤ 7, n ≤ 12. Reproducible intensity anomalies are observed in high-resolution mass spectra. For both charge states, (C60)mCs3± and (C60)mCs5± are particularly abundant, with little dependence on the value of m. Distributions of bare cesium cluster ions also indicate enhanced stability of Cs3± and Cs5±, in agreement with theoretical predictions. These findings contrast with earlier reports on highly Cs-doped cationic fullerene aggregates which showed enhanced stability of C60Cs6 building blocks attributed to charge transfer. The dependence of the (C60)mCs3– anion yield on electron energy shows a resonance that, surprisingly, oscillates in strength as m increases from 1 to 6.

Complexes of gold and imidazole formed in helium nanodroplets

We have studied complexes of gold atoms and imidazole (C3N2H4) produced in helium nanodroplets.

Highly Stable [C60AuC60]+/- Dumbbells

Ionic complexes between gold and C60 have been observed for the first time. Cations and anions of the type [Au(C60)2]+/– are shown to have particular stability. Calculations suggest that these ions adopt a C60–Au–C60 sandwich-like (dumbbell) structure, which is reminiscent of [XAuX]+/– ions previously observed for much smaller ligands. The [Au(C60)2]+/– ions can be regarded as Au(I) complexes, regardless of whether the net charge is positive or negative, but in both cases, the charge transfer between the Au and C60 is incomplete, most likely because of a covalent contribution to the Au–C60 binding. The C60–Au–C60 dumbbell structure represents a new architecture in fullerene chemistry that might be replicable in synthetic nanostructures.

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.

Electron-induced chemistry in imidazole clusters embedded in helium nanodroplets

Abstract 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. Graphical abstract

Low-temperature Condensation of Carbon

Two different types of experiments were performed. In the first experiment, we studied the low-temperature condensation of vaporized graphite inside bulk liquid helium, while in the second experiment, we studied the condensation of single carbon atoms together with H

Cs+ Solvated in Hydrogen—Evidence for Several Distinct Solvation Shells

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Ordered phases of ethylene adsorbed on charged fullerenes and their aggregates.

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