Ca+ Ions Solvated in Helium Clusters

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.

Molecular Oxygen Trimer: Multiplet Structures and Stability

“…In practice, we have used the code developed by Sandler and Buch, [31] previously employed in calculations of the (O 2 ) 3 septet state [26] as well as in studies of He or H 2 clusters doped with atomic ions. [33][34][35] For each value of the total spin, we have performed calculations using N = 10 4 replicas, initially built from a Gaussian distribution centered on the minima structures previously obtained. An initial propagation up to t max ¼5�10 5 a.u.…”

Observation of Multiple Ordered Solvation Shells in Doped Helium Droplets: The Case of HeNCa2+

“…7 The strength of the He−Ca + interaction (4.6 meV, 51 see Figure S2 of the Supporting Information), smaller than the presently investigated He−Ca 2+ case (154.6 meV), is also responsible of differences in the corresponding solvated He ions: As opposed to He N Ca 2+ , where abrupt steps in the ion abundance correspond to welldefined structures, the He N Ca + complexes seem to present a gradual closing of the first shell between N = 17 and N = 25 compatible with a liquid-like nature of the clusters. 51 HNDs doped with Kr 2+ could be good candidates to exhibit similar large ordered structures, since magic numbers at N = 12 and 32 have been already observed in the spectra. 52 Moreover, preliminary calculations reveal that the electronic ground state He-Kr 2+ ( 3 P 2 ) could be described with an interparticle equilibrium distance at the same range as He−Ca 2+ with an even deeper potential well depth.…”

“…50 In turn, for He N Pb 2+ an icosahedral structure has been suggested at N = 12. 7 The strength of the He−Ca + interaction (4.6 meV, 51 see Figure S2 of the Supporting Information), smaller than the presently investigated He−Ca 2+ case (154.6 meV), is also responsible of differences in the corresponding solvated He ions: As opposed to He N Ca 2+ , where abrupt steps in the ion abundance correspond to welldefined structures, the He N Ca + complexes seem to present a gradual closing of the first shell between N = 17 and N = 25 compatible with a liquid-like nature of the clusters. 51 HNDs doped with Kr 2+ could be good candidates to exhibit similar large ordered structures, since magic numbers at N = 12 and 32 have been already observed in the spectra.…”

Intermolecular Forces: From Atoms and Molecules to Nanostructures

“…Specifically, they analyzed how the modeling results change in connection with the polarizable continuum model (PCM) and with the use of the Adduct under Field (AuF) approach to nonpolar solvents, and how the AuF approach can simulate the dissociation of alkali metal halides. The solvation of Ca + with helium has been investigated at the atomic level by Bartolomei et al [ 7 ] in a combined experimental and theoretical work, which employs quantum Monte Carlo calculations and basin-hopping structure optimization based on the improved Lennard–Jones (ILJ) pair potential, while a reflectron time-of-flight mass spectrometer was applied to analyze the product ions ejected. These authors concluded that 25 helium atoms complete the first solvation shell (the optimum packing of He N Ca + is ), and the corresponding clusters are essentially liquid-like structures.…”