Effects of resonant coupling on the formation of ultracold $^{85}$Rb$_2$ molecules

We have studied the effect of resonant electronic state coupling on the formation of ultracold ground-state $^{85}$Rb$_2 $. The ultracold Rb$_2$ are formed by photoassociation (PA) to the $0_u^+$ state converging to the $5S+5P_{1/2}$ limit, followed by radiative decay into high vibrational levels of the ground state, $X~^1\Sigma_g^+$. The populations of high-$v$ levels of the $X$ state are monitored by resonance-enhanced two- photon ionization through the $2~^1\Sigma_u^+$ state. We find that the populations of vibrational levels $v"$=112-116 are far larger than can be accounted for by the Frank-Condon factors for $0_u^+ \leftarrow X~^1\Sigma_g^+$ transitions. Further, the ground-state molecule population exhibits oscillatory behavior as the PA laser is tuned through a succession of $0_u^+$ state vibrational levels. Both of these effects explained by a new calculation of transition amplitudes that includes the resonant character of the spin-orbit coupling of the two $0_u^+$ states converging to the $5P_{1/2}$ and $5P_{3/2}$ limits. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules. We also present the progress toward forming $^{85}$Rb$_2$ by photoassociation in an optical dipole trap using a CO$_2$ laser. This work is supported by the NSF.