Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe.

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms. Under gentle stripping conditions, helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions, some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters of NgAg at the expense of less stable ones. This results in local anomalies in the cluster ion abundance, which is measured in a high-resolution time-of-flight mass spectrometer. On the basis of these data, we identify specific "magic" sizes of particularly stable ions. There is no evidence, however, for enhanced stability of NgAg, in contrast to the high stability of NgAu that derives from the covalent nature of the bond for heavy noble gases. "Magic" sizes are also identified for Ag dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for HeHO.