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

By means of the Path Integral Monte Carlo method, we have performed a detailed microscopic study of ⁴He 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 three concentric shells in which the ⁴He atoms are placed 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 118:8050, 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 Galli et al. (J Phys Chem A 115:7300, 2011) in a study regarding Na⁺@⁴He<inf>n</inf> when n>30; a comparison between Ar⁺@⁴He<inf>n</inf> and Na⁺@⁴He<inf>n</inf> complexes is also presented.