Single-color photoassociative ionization of ultracold sodium: The region from 0 to -5 GHz

We have measured the rate of production of Na₂⁺ ions in collisions of ultracold Na atoms held in a magneto-optical trap (MOT) as a function of probe laser detuning using a single-color probe beam. The ion rate is measured with the atoms mainly in the $3S(f=2)$ level (``bright MOT'') or the $3S(f=1)$ level (``dark MOT''). Using recent experimental information about the doubly excited autoionizing states of Na₂, we find that the large structures in the first 5 GHz red of the atomic transition frequency are due to a doubly resonant excitation process with levels in the 0g^- potential (asymptotically connected to 3S1/2+3P3/2) as the intermediate states, and levels in the 0ᵤ^- and 1ᵤ potentials (asymptotically connected to 3P3/2+3P3/2) as the final autoionizing states. We can account for nearly all of the observed structure in this region of the spectra for both the bright and dark MOT.