Hyperfine, rotational, and vibrational structure of the a3Σu+ state of 87Rb2

We have performed high-resolution two-photon dark-state spectroscopy of an ultracold gas of 87Rb2 molecules in the a3Σu+ state at a magnetic field of about 1000 G. The vibrational ladder as well as the hyperfine and low-lying rotational structure are mapped out. Energy shifts in the spectrum are observed due to singlet-triplet mixing at binding energies as deep as a few hundred GHzxh. This information, together with data from other sources, is used to optimize the potentials of the a3Σu+ and X1Σg+ states in a coupled-channel model. We find that the hyperfine structure depends weakly on the vibrational level. This provides a possible explanation for inaccuracies in recent Feshbach resonance calculations.