Path Integral Monte Carlo Study Confirms a Highly Ordered Snowball in 4He Nanodroplets Doped with an Ar+ Ion

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 4 He 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 4 He 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 [1] 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.[2] 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.

Adsorption of sodium and cesium on aggregates of C60

“…For example, the abundance distribution of ArHe + n which reveals three anomalies [50] is in striking agreement with computed dissociation energies [51]. Unfortunately, the only system for which energies of (C 60 ) m Cs z+ n have been computed versus a continuous range of n values, thus allowing the extraction of D n values, is C 60 Li z+ n with z = 0, 1, 2, and n 14 [3].…”

Helium nanodroplets doped with copper and water

“…the ligand-ligand bond length. For He n Ar + the match is near-perfect, resulting in a highly ordered system with three distinct solvation shells of I h symmetry [48].…”

Atomically resolved phase transition of fullerene cations solvated in helium droplets

“…The size of this shell can be determined from distinctive steps in the dissociation energy and thus of steps in the ion abundance measured in mass spectrometers. The size may be 12 helium atoms for small cations, such as Ar + (refs 22, 23) and up to 60 or 62 atoms for C 60 + or C 70 + fullerene cations, respectively2425. Although these so-called magic number effects are well known, their relation to Atkins snowballs and the onset and extent of superfluidity remains obscured.…”