Enhancement of the formation of ultracold Rb285 molecules due to resonant coupling

We have studied the effect of resonant electronic-state coupling on the formation of ultracold ground-state $^{85}\mathrm{Rb}_{2}$. Ultracold ${\mathrm{Rb}}_{2}$ molecules are formed by photoassociation (PA) to a coupled pair of ${0}_{u}^{+}$ states, ${0}_{u}^{+}({P}_{1∕2})$ and ${0}_{u}^{+}({P}_{3∕2})$, in the region below the $5S+5{P}_{1∕2}$ limit. Subsequent radiative decay produces high vibrational levels of the ground state, $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$. The population distribution of these $X$-state vibrational levels is monitored by resonance-enhanced two-photon ionization through the $2\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{u}^{+}$ state. We find that the populations of vibrational levels ${v}^{\ensuremath{''}}=112--116$ are far larger than can be accounted for by the Franck-Condon factors for ${0}_{u}^{+}({P}_{1∕2})\ensuremath{\rightarrow}X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}_{g}^{+}$ transitions with the ${0}_{u}^{+}({P}_{1∕2})$ 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 ${0}_{u}^{+}$ 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 ${0}_{u}^{+}$ states. The resulting enhancement of more deeply bound ground-state molecule formation will be useful for future experiments on ultracold molecules.