Enhancement of the formation of ultracold {sup 85}Rb{sub 2} molecules due to resonant coupling

We have studied the effect of resonant electronic-state coupling on the formation of ultracold ground-state {sup 85}Rb{sub 2}. Ultracold Rb{sub 2} molecules are formed by photoassociation (PA) to a coupled pair of 0{sub u}{sup +} states, 0{sub u}{sup +}(P{sub 1/2}) and 0{sub u}{sup +}(P{sub 3/2}), in the region below the 5S+5P{sub 1/2} limit. Subsequent radiative decay produces high vibrational levels of the ground state, X {sup 1}{sigma}{sub g}{sup +}. The population distribution of these X-state vibrational levels is monitored by resonance-enhanced two-photon ionization through the 2 {sup 1}{sigma}{sub u}{sup +} state. We find that the populations of vibrational levels v{sup ''}=112-116 are far larger than can be accounted for by the Franck-Condon factors for 0{sub u}{sup +}(P{sub 1/2}){yields}X {sup 1}{sigma}{sub g}{sup +} transitions with the 0{sub u}{sup +}(P{sub 1/2}) state treated as a single channel. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of 0{sub u}{sup +} state vibrational levels. Both of these effects are explained by a calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two 0{sub u}{sup +} states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.