H. K. PECHKIS, D. WANG, Y. HUANG, E. E. EYLER, P. L. GOULD AND W. C. STWALLEY, Physics Department, University of Connecticut, Storrs, CT 06269; C. P. KOCH, Institut für Theoretische Physik, Freie Universität Berlin, 14195 Berlin, Germany.

We describe experimental and theoretical studies of the effects of resonant electronic state coupling on the formation of ultracold ground-state 85Rb2. The molecules are formed by photoassociation of ultracold atoms in a MOT into the 0u+ state converging to the 5S+5P1/2 limit, followed by radiative decay into high vibrational levels of the ground electronic state, X 1 Sigmag+. The populations of these high-v ground-state levels are monitored by resonance-enhanced two-photon ionization (R2PI) through the 2 1 Sigmau+ state. We find that the populations of vibrational levels v''=112-116 are far larger than can be accounted for by the Franck-Condon factors for 0u+ leftarrow X 1 Sigmag+ transitions. Further, the total number of ground-state molecules formed by this process exhibits oscillatory behavior as the PA laser is tuned through a succession of 0u+ state vibrational levels. Both of these effects are explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling between the two 0u+ states converging to the 5P1/2 and 5P3/2 limits. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.

We also describe evidence from our R2PI spectra for extensive singlet-triplet mixing between excited states of Rb2 at intermediate internuclear separations, apparently also induced by spin-orbit interactions. In particular, the 3 1 Sigmag+ and 1 1 Deltag states converging to 5s+4d have been observed in excitation from the a 3 Sigmau+ state, and the 2 3 Piu state has been observed in excitation from the X 1 Sigmag+ state.