  We have performed high-resolution two-photon dark-state spectroscopy of an
ultracold gas of Rb molecules in the triplet ground state at a magnetic field
of about 1000 G. The vibrational ladder as well as the hyperfine and low-lying
rotational structure is mapped out. Energy shifts in the spectrum are observed
due to singlet-triplet mixing at binding energies as deep as a few hundred GHz
x h. This information together with data from other sources is used to optimize
the potentials of the triplet and singlet 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.
