Robert Moszynski, Michał Krych, Wojciech Skomorowski, Filip Pawłowski, and Zbigniew Idziaszek
Departament of Chemistry and Department of Physics, University of Warsaw, Poland

State-of-the-art ab initio techniques have been applied to compute the potential energy curves of the (BaRb)⁺ molecular ion in the Born-Oppenheimer approximation for the singlet and triplet states dissociating into the ground state 1S Rb⁺ ion and the Ba atom in the ground ¹S state, the lowest singlet or triplet D excited states, and for the singlet and triplet states dissociating into the ground state ²S Rb atom and the ground state ²S Ba⁺ ion. The ground state potential energy was obtained with the coupled cluster method restricted to single, double, and nonperturbative triple excitations. The first triplet states in the Σ, Π, and Δ symmetries were computed with the restricted open-shell coupled cluster method restricted to single, double, and nonperturbative triple excitations. All other excited state potential energy curves were computed using the equation of motion approach within the coupled-cluster singles, doubles, and linear triples framework. The long-range coefficients describing the electrostatic, induction, and dispersion interactions at large interatomic distances are also reported. The electric transition dipole moments governing the X¹Σ → ¹Σ, ¹Π have been obtained as the first residue of the polarization propagator computed with the linear response coupled-cluster method restricted to single and double excitations. Nonadiabatic radial and angular coupling matrix elements, as well as the spin-orbit coupling matrix elements have been evaluated using the multireference configuration interaction method restricted to single and double excitations with a large active space. With these couplings, the spin-orbit coupled (relativistic) potential energy curves for the 0⁺ and 1 states relevant for the running experiments have been obtained. Finally, relativistic transition moments and nonadiabatic coupling matrix elements were obtained from the nonrelativistic results and spin-orbit eigenvectors. The electronic structure input has been employed in the single channel scattering calculations of the collisional cross sections between the Ba+ ion and Rb atom. Both nonrelativistic and relativistic potentials were used in these calculations. Our results show that the inelastic cross section corresponding to the charge transfer from the Rb atom to the Ba⁺ ion is much smaller than the elastic one over a wide range of energies up to 1 mK. This suggests that sympathetic cooling of the Ba⁺ ion by collisions with ultracold Rb atoms should be possible.