Optical double resonance photoassociation spectrum of ground state Rb_2

We report the first observation of the highest lying bound levels of the ground electronic state of ^85Rb2 by means of optical double resonance (free-bound-bound) photoassociation spectroscopy of colliding, laser-cooled ^85Rb atoms. Twelve levels with binding energies from 0.13 to 20 Ghz are observed. We carry out an inverse perturbation analysis of these levels that takes full account of the hyperfine-induced singlet-triplet mixing with a coupled channels approach. This analysis yields previously undetermined atomic interaction parameters that are crucial for the understanding of Rb ultracold collisions and Bose-Einstein condensates. Complete consistency is obtained with all previously measured cold collision properties of Rb. We gratefully acknowledge the support at Texas of the R. A. Welch Foundation, the NSF, and the Microgravity Science and Applications Division of NASA