Trapping molecular ions formed via photo-associative ionization of ultracold atoms

The formation of ${}^{40}$Ca${}_2^{+}$ molecular ions is observed in a hybrid ${}^{40}$Ca magneto-optical and ion trap system. The molecular ion formation process is determined to be two-photon photo-associative ionization of ultracold ${}^{40}$Ca atoms. A lower bound for the two-body, two-photon rate constant is found to be $\bar{\beta }\ge 2\pm 1\times {10}^{-15}$ cm${}^3$ Hz. $\mathit{\text{Ab}}\mathit{\text{initio}}$ molecular potential curves are calculated for the neutral Ca${}_2$ and ionic Ca${}_2^{+}$ molecules and used in a model that identifies the photo-associative ionization pathway. As this technique does not require a separate photo-association laser, it could find use as a simple, robust method for producing ultracold, state-selected molecular ions.