The interaction of He− with fullerenes

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Atomic Gold Ions Clustered with Noble Gases: Helium, Neon, Argon, Krypton, and Xenon

High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal HenAu+ cluster ions. Additional doping with heavy noble gases results in NenAu+, ArnAu+, KrnAu+, and XenAu+ cluster ions. The high stability predicted for covalently bonded Ar2Au+, Kr2Au+, and Xe2Au+ is reflected in their relatively high abundance. Surprisingly, the abundance of Ne2Au+, which is predicted to have zero covalent bonding character and no enhanced stability, features a local maximum, too. The predicted size and structure of complete solvation shells surrounding ions with essentially nondirectional bonding depends primarily on the ratio σ* of the ion-ligand versus the ligand-ligand distance. For Au+ solvated in helium and neon, the ratio σ* is slightly below 1, favoring icosahedral packing in agreement with a maximum observed in the corresponding abundance distributions at n = 12. HenAu+ appears to adopt two additional solvation shells of Ih symmetry, containing 20 and 12 atoms, respectively. For ArnAu+, with σ* ≈ 0.67, one would expect a solvation shell of octahedral symmetry, in agreement with an enhanced ion abundance at n = 6. Another anomaly in the ion abundance at Ar9Au+ matches a local maximum in its computed dissociation energy.

Hydrogenated Gold Clusters from Helium Nanodroplets: Cluster Ionization and Affinities for Protons and Hydrogen Molecules

We report the mass spectrometric detection of hydrogenated gold clusters ionized by electron transfer and proton transfer. The cations appear after the pickup of hydrogen molecules and gold atoms by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. We focus on the size distributions of the gold cluster cations and their hydrogen content, the electron energy dependence of the ion yield, patterns of hydrogenated gold cluster cation stability, and the presence of "magic" clusters. Ab initio molecular orbital calculations were performed to provide insight into ionization energies and proton affinities of gold clusters as well as into molecular hydrogen affinities of the ionized and protonated gold cluster cations.

Charged Clusters of C60 and Au or Cu: Evidence for Stable Sizes and Specific Dissociation Channels

We have doped helium nanodroplets with C60 and either gold or copper. Positively or negatively charged (C60) mM n± ions (M = Au or Cu) containing up to ≈10 fullerenes and ≈20 metal atoms are formed by electron ionization. The abundance distributions extracted from high-resolution mass spectra reveal several local anomalies. The sizes of the four most stable (C60) mAu n± ions identified in previous calculations for small values of m and n ( m ≤ 2 and n ≤ 2, or m = 1 and n = 3) agree with local maxima in the abundance distributions. Our data suggest the existence of several other relatively stable ions including (C60)2Au3± and (C60)3Au4-. Another feature, namely the absence of bare (C60)2±, confirms the prediction that (C60)2M± dissociates by loss of C60± rather than loss of M. The experimental data also reveal the preference for loss of (charged or neutral) C60 over loss of a metal atom from some larger species such as (C60)3M3+. In contrast to these similarities between Au and Cu, the abundance distributions of (C60)3Au n- and (C60)3Cu n- are markedly different. In this discussion, we emphasize the similarities and differences between anions and cations, and between gold and copper. Also noteworthy is the observation of dianions (C60) mAu n2- for m = 2, 4, and 6.

Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets

We report the observation of sequential encounters of fullerenes with C atoms in an extremely cold environment. Experiments were performed with helium droplets at 0.37 K doped with C60 molecules and C atoms derived from a novel, pure source of C atoms. Very high-resolution mass spectra revealed the formation of carbenes of the type C60(C:)n with n up to 6. Bridge-type bonding of the C adatoms to form the known dumbbell C60═C═C60 also was observed. Density functional theory calculations were performed that elucidated the carbene character of the C60(C:)n species and their structures. Mass spectra taken in the presence of water impurities and in separate experiments with added H2 also revealed the formation of the adducts C60C(n)(H2O)n and C60C(n)(H2)n probably by H-OH and H-H bond insertion, respectively, and nonreactivity for the dumbell. So C adatoms that form carbenes C60(C:)n can endow pristine C60 with a higher chemical reactivity.