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

Abstract

High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal He n Au+ cluster ions. Additional doping with heavy noble gases results in Ne n Au+, Ar n Au+, Kr n Au+, and Xe n Au+ 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. He n Au+ appears to adopt two additional solvation shells of I h symmetry, containing 20 and 12 atoms, respectively. For Ar n Au+, 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.

Complexes with Atomic Gold Ions: Efficient Bis-Ligand Formation

“…A pronounced maximum at n = 12 was explained by the completion of an icosahedral shell and a local maximum at n = 14 by another stable structure where two additional helium atoms can be squeezed into the first shell, similar to He n Kr + , where also n = 14 turned out to be magic [38]. Pronounced magic numbers were also reported in a subsequent study where HNDs were doped with gold vapor and another rare gas Rg (Rg = Ne, Ar, Kr or Xe) [14]. For all four noble gases, the intensity of Rg 2 Au + exhibits a local maximum which agrees in the case of Ar, Kr and Xe with a covalent character of the bonding [6].…”

“…The ligand most weakly bound to gold is helium [9] and thus ligand-exchange with every other ligand can be expected inside a helium nanodroplet (HND). In the case of neutral helium droplets, this was successfully demonstrated by the growth of pristine and mixed gold clusters containing a few gold atoms [10][11][12][13][14][15][16][17][18], as well as nanoparticles and nanowires that contain many thousand atoms [19][20][21][22][23][24][25]. Recently, we doped neutral HNDs containing about 10 6 He atoms with gold vapor and investigated low-mass ions formed upon electron ionization and ejected from the massive droplets [12].…”

“…Pre-doping of charged HNDs with hydrogen was successfully utilized to prevent fragmentation of fragile molecular dopants by a gentle proton transfer reaction and avoid the exothermic charge transfer process from He n + [36,37]. Besides pristine gold cluster ions Au m + , binary cluster ions of the form He n Au m + were also observed [12,14]. For He n Au + , in excess of n = 100 helium atoms were seen.…”

“…Pronounced enhancement of the bis-ligand structures is observed for Xe, Kr, and Ar, little enhancement for Ne, no enhancement for He. Additional intensity anomalies at n = 4, 6, and 12 indicate shell closures that can be assigned to tetrahedral, octahedral and icosahedral solvation layers, respectively [14].…”

“…Ar shows two less pronounced maxima at n = 6 and 9 and Kr a weak one at n = 6. Based on a hard-sphere model a maximum at n = 6 can be assigned to an octahedral solvation shell [14]. The intensity drop at n = 4 in the case of Xe is fairly weak; it does not necessarily indicate a tetrahedral shell closure.…”