Production of NaCa+ molecular ions in the ground state from cold atom-ion mixtures by photoassociation via an intermediate state

We present a theoretical analysis of optical pathways for formation of cold ground-state (NaCa)(+) molecular ions via an intermediate state. The formation schemes are based on ab initio potential energy curves and transition dipole moments calculated using effective-core-potential methods of quantum chemistry. In the proposed approach, starting from a mixture of cold trapped Ca+ ions immersed into an ultracold gas of Na atoms, (NaCa)(+) molecular ions are photoassociated in the excited E-1 Sigma(+) electronic state and allowed to spontaneously decay either to the ground electronic state or an intermediate state from which the population is transferred to the ground state via an additional optical excitation. By analyzing all possible pathways, we find that the efficiency of a two-photon scheme, via either the B-1 Sigma(+) or C-1 Sigma(+) potential, is sufficient to produce significant quantities of ground-state (NaCa)(+) molecular ions. A single-step process results in lower formation rates that would require either a high-density sample or a very intense photoassociation laser to be viable.