Two-color photoassociation spectroscopy of ground state Rb-2

We determine the energies of twelve vibrational levels lying within 20 GHz of the lowest dissociation limit of Rb-85(2) with two-color photoassociation spectroscopy of ultracold Rb-85 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 Rb-85(2) interaction parameters. The information obtained is sufficient to allow for quantitative calculations of arbitrary Rb ultracold collision properties.