The adsorption of helium atoms on small cationic gold clusters

Abstract

Adducts formed between small gold cluster cations and helium atoms are reported for the first time. These binary ions, Au_n⁺He_m, were produced by electron ionization of helium nanodroplets doped with neutral gold clusters and were detected using mass spectrometry. For a given value of n, the distribution of ions as a function of the number of added helium atoms, m, has been recorded. Peaks with anomalously high intensities, corresponding to so-called magic number ions, are identified and interpreted in terms of the geometric structures of the underlying Au_n⁺ ions. These features can be accounted for by planar structures for Au_n⁺ ions with n ≤ 7, with the addition of helium having no significant effect on the structures of the underlying gold cluster ions. According to ion mobility studies and some theoretical predictions, a 3-D structure is expected for Au₈⁺. However, the findings for Au₈⁺ in this work are more consistent with a planar structure.