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

TL;DR

This study introduces a novel method to investigate the solvation of silver ions in noble gas clusters using helium nanodroplets, revealing specific stable cluster sizes and contrasting stability patterns with gold ions.

Contribution

The paper presents a new experimental technique for forming and analyzing silver ion-noble gas clusters, identifying stable 'magic' sizes and comparing stability with gold ion complexes.

Findings

Identification of 'magic' cluster sizes indicating stability.

No enhanced stability observed for Ng$_2$Ag$^+$ compared to Ng$_2$Au$^+$.

Detection of stable Ag$_2^+$ complexes with He and Kr.

Abstract

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves 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 Ng$_n$Ag$_m^+$ 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. Based on these data we identify specific "magic" sizes n of particularly stable ions. There is no evidence though for enhanced stability of Ng$_2$Ag$^+$, in contrast to the high stability of Ng$_2$Au$^+$ that derives from the covalent nature of the bond for heavy noble gases. "Magic" sizes are also identified for Ag$_2^+$ dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers $n$ = 12, 32, 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for He$_n$H$_2$O$^+$.