We report here on the production of an ultracold gas of tightly bound Rb2 triplet molecules in the rovibrational ground state, close to quantum degeneracy. This is achieved by optically transferring weakly bound Rb2 molecules to the absolute lowest level of the ground triplet potential with a transfer efficiency of about 90%. The transfer takes place in a 3D optical lattice which traps a sizeable fraction of the tightly bound molecules with a lifetime exceeding 200 ms.

We have performed high-resolution two-photon dark-state spectroscopy of an ultracold gas of 87 Rb2 molecules in the a 3 Σ + u 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×h. This information together with data from other sources is used to optimize the potentials of the a 3 Σ + u and X 1 Σ + g 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.

We present the results of an experimental and theoretical study of the electronically excited (1) 3 + g state of 87 Rb 2 molecules. The vibrational energies are measured for deeply bound states from the bottom up to v = 15 using laser spectroscopy of ultracold Rb 2 Feshbach molecules. The spectrum of each vibrational state is dominated by a 47-GHz splitting into 0 − g and 1 g components caused mainly by a strong second-order spin-orbit interaction. Our spectroscopy fully resolves the rotational, hyperfine, and Zeeman structure of the spectrum. We are able to describe this structure to the first order using a simplified effective Hamiltonian.