Solvation of Intrinsic Positive Charge in Superfluid Helium

Abstract

Based on electronic structure calculations, the structure of intrinsic positive charge solvated in superfluid helium is identified as triatomic He3^+ ion, which is bound to the surrounding ground state helium atoms through the charge - charge induced dipole interaction in a pairwise additive manner. Bosonic density functional theory calculations show that this ion forms the well-known Atkins' snowball solvation structure where the first rigid helium shell is effectively disconnected from the rest of the liquid. Evaluation of the total energy vs. helium droplet size N shows distinct regions related to the completion of solvent shells near N=16 and N=47. These regions can be assigned to magic numbers observed in positively charged helium droplets appearing at N=15 and in the range between 20-50 helium atoms. The calculated added mass for the positive ion in bulk superfluid helium (18 m_He) is much smaller than the previous experiments suggest (30-40 m_He) indicating that there may be yet some unidentified additional factor contributing to the measured effective mass. Both previous experiments and the present calculations agree on the effective mass of the negative charge (240-250 m_He). The main difference between the solvated negative and positive charges in liquid helium is that the latter forms a chemically bound triatomic molecule surrounded by highly inhomogeneous liquid structure whereas the former remains as a separated charge with a smoothly varying liquid density around it.