  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.
