Observation and Analysis of Resonant Coupling Between Near-Degenerate Levels of the 2 1\Sigma g+ and 1 1 \Pi g States of Ultracold 85Rb2

We report on the anomalously high line strength of a single rotational level in the ultracold photoassociation of two 85Rb atoms to form 85Rb2. The v' = 111, J' = 5 level belongs to the Hund's case (c) 2 (0g+) state, which correlates to the Hund's case (a) 2 1 \Sigma g+ state. Its strength is caused by coupling with a very near-resonant long-range state. The long-range component is the energetically degenerate v' = 155, J' = 5 level of the case (c) 2 (1g)$ state, correlating to the case (a) 1 1 \Pi g state. The line strength is enhanced by an order of magnitude through this coupling, relative to nearby vibrational levels and even to nearby rotational levels of the same vibrational level. This enhancement is in addition to the enhancement seen in all J' = 3 and 5 levels of the 2 (0g+) state due to an l = 4 shape resonance in the a 3 \Sigma u+ state continuum, which alters the distribution of levels formed by photoassociation.