Ca(+) Ions Solvated in Helium Clusters

We present a combined experimental and theoretical investigation on Ca [Formula: see text] ions in helium droplets, He [Formula: see text] Ca [Formula: see text]. 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 fine-tuned by exploiting ab initio estimations. Ion yields of He [Formula: see text] Ca [Formula: see text] -obtained via high-resolution mass spectrometry- generally decrease with N with a more pronounced drop between [Formula: see text] and [Formula: see text] , the computed quantum He [Formula: see text] Ca [Formula: see text] evaporation energies resembling this behavior. The analysis of the energies and structures reveals that covering Ca [Formula: see text] 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 [Formula: see text] , which corresponds to the maximum number of atoms in that shell. Behavior of the evaporation energies and radial densities suggests liquid-like cluster structures.