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_nAu⁺ cluster ions. Additional doping with heavy noble gases results in Ne_nAu⁺, Ar_nAu⁺, Kr_nAu⁺, and Xe_nAu⁺ 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_nAu⁺ appears to adopt two additional solvation shells of I_h symmetry, containing 20 and 12 atoms, respectively. For Ar_nAu⁺, 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.