Accurate calculations of the ground state and low-lying excited states of the (RbBa)+ molecular ion: a proposed system for ultracold reactive collisions

Collisions of ultracold Ba+ ions on a Rb Bose–Einstein condensate have been suggested as a possible benchmark system for ultracold ion-neutral collision experiments. However, a priori knowledge of the possible processes is desirable. For this purpose, we here present high-level four-component coupled cluster and multi-reference configuration interaction calculations of potential energy curves, dipole moments and spectroscopic constants of the experimentally interesting low-lying electronic states of the (RbBa)+ molecule. Our results show significant avoided crossings between the Rb + Ba+ entrance channels and low-lying charge transfer states of the Rb+ and Ba(3D) atomic channels, indicating that a fast non-radiative charge transfer is possible. Population analysis shows that a partially covalent polar bond is formed in the ground state, which thus deviates significantly from a pure Rb+ + Ba interaction. This finding is corroborated by the electric dipole moment which is found only to be 4.5 D at the equilibrium bond distance, compared with the 14 D for a pure Rb+ + Ba interaction, thereby supporting the view of a partial charge transfer between the two atoms.