Path Integral Monte Carlo study confirms a highly ordered snowball in 4He nanodroplets doped with an Ar+ ion

TL;DR

This study uses Path Integral Monte Carlo simulations to reveal a highly ordered, multi-shell snowball structure of helium atoms around an Ar+ ion in nanodroplets, confirming experimental observations and comparing with similar sodium ion complexes.

Contribution

It provides the first detailed microscopic characterization of the ordered snowball structure around Ar+ in helium nanodroplets, aligning theory with recent experimental findings.

Findings

Identification of a three-shell snowball structure with platonic solid arrangements.

Correlation of dissociation energy jumps with magic numbers 12, 32, and 44.

Comparison showing similar structures in Na+ and Ar+ helium complexes.

Abstract

By means of the exact Path Integral Monte Carlo method we have performed a detailed microscopic study of $^4$He nanodroplets doped with an argon ion, Ar$^+$, at $T=0.5$ K. We have computed density profiles, energies, dissociation energies and characterized the local order around the ion for nanodroplets with a number of 4He atoms ranging from 10 to 64 and also 128. We have found the formation of a stable solid structure around the ion, a "snowball", consisting of 3 concentric shells in which the 4He atoms are placed on at the vertices of platonic solids: the first inner shell is an icosahedron (12 atoms); the second one is a dodecahedron with 20 atoms placed on the faces of the icosahedron of the first shell; the third shell is again an icosahedron composed of 12 atoms placed on the faces of the dodecahedron of the second shell. The "magic numbers" implied by this structure, 12, 32 and 44 helium atoms, have been observed in a recent experimental study [Bartl et al, J. Phys. Chem. A 118, 2014] of these complexes; the dissociation energy curve computed in the present work shows jumps in correspondence with those found in the nanodroplets abundance distribution measured in that experiment, strengthening the agreement between theory and experiment. The same structures were predicted in Ref. [Galli et al, J. Phys. Chem. A 115, 2011] in a study regarding Na$^+$@$^4$He$_n$ when n>30; a comparison between Ar$^+$@$^4$He$_n$ and Na$^+$@$^4$He$_n$ complexes is also presented.