Enhancement of the formation of ultracold ⁸⁵Rb₂ molecules due to resonant coupling

We have studied the effect of resonant electronic-state coupling on the formation of ultracold ground-state ⁸⁵Rb₂. Ultracold Rb₂ 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+5P_1∕2 limit. Subsequent radiative decay produces high vibrational levels of the ground state, X ¹Σ_g⁺. The population distribution of these X-state vibrational levels is monitored by resonance-enhanced two-photon ionization through the 2 ¹Σ_u⁺ 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 0_u⁺(P_1∕2)→X ¹Σ_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.