Ca+ Ions Solvated in Helium Clusters

13 pags., 6 figs. -- This article belongs to the Special Issue Intermolecular Forces: From Atoms and Molecules to NanostructuresWe present a combined experimental and theoretical investigation on Ca+ ions in helium droplets, HeNCa+. The clusters have been formed in the laboratory by means of electron-impact ionization of Ca-doped helium nanodroplets. Energies and structures of such complexes have been computed using various approaches such as path integral Monte Carlo, diffusion Monte Carlo and basin-hopping methods. The potential energy functions employed in these calculations consist of analytical expressions following an improved Lennard-Jones formula whose parameters are finetuned by exploiting ab initio estimations. Ion yields of HeNCa+ -obtained via high-resolution mass spectrometry- generally decrease with N with a more pronounced drop between N = 17 and N = 25, the computed quantum HeNCa+ evaporation energies resembling this behavior. The analysis of the energies and structures reveals that covering Ca+ with 17 He atoms leads to a cluster with one of the smallest energies per atom. As new atoms are added, they continue to fill the first shell at the expense of reducing its stability, until N = 25, which corresponds to the maximum number of atoms in that shell. Behavior of the evaporation energies and radial densities suggests liquid-like cluster structuresThis research was funded by the Austrian Science Fund FWF project number P31149 (P.M., P.S.) and the Spanish MICINN with Grants FIS2016-79596-P and PID2019-105225GB-I00 (J.B., J.H.-R.), FIS2017-83157-P (T.G.-L.) and FIS2017-84391-C2-2-P (M.B., J.C.-M., M.I.H.).Peer reviewe