Two-color photoassociation spectroscopy of ground state Rb₂

We determine the energies of twelve vibrational levels lying within 20 GHz of the lowest dissociation limit of ⁸⁵Rb₂with two-color photoassociation spectroscopy of ultracold ⁸⁵Rb atoms. The levels lie in an energy range for which singlet and triplet states are mixed by the hyperfine interaction. We carry out a coupled channels bound state analysis of the level energies, and derive accurate values for ⁸⁵Rb₂interaction parameters. The information obtained is sufficient to allow for quantitative calculations of arbitrary Rb ultracold collision properties.