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Ca+ Ions Solvated in Helium Clusters
Authors:Massimiliano Bartolomei  Paul Martini  Ricardo Prez de Tudela  Toms Gonzlez-Lezana  Marta I Hernndez  Jos Campos-Martínez  Javier Hernndez-Rojas  Jos Bretn  Paul Scheier
Institution:1.Instituto de Física Fundamental (IFF-CSIC), Serrano 123, 28006 Madrid, Spain; (M.B.); (J.C.-M.);2.Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, Technikerstr. 25, A-6020 Innsbruck, Austria;3.Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, 44780 Bochum, Germany;4.Departamento de Física and IUdEA, Universidad de La Laguna, 38205 Tenerife, Spain; (J.H.-R.); (J.B.)
Abstract:We 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 fine-tuned 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 structures.
Keywords:molecular clusters  solvation  helium-alkaline earth ion interactions  helium nanodroplets  mass spectrometry  classical/quantum monte carlo calculations
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