High-resolution molecular spectroscopy for producing ultracold absolute-ground-state 23Na87Rb molecules

We report a detailed molecular spectroscopy study of the lowest excited electronic states of Na-23 Rb-87 for producing ultracold Na-23 Rb-87 molecules in the electronic, rovibrational, and hyperfine ground state. Starting from weakly bound Feshbach molecules, a series of vibrational levels of the A(1) Sigma(+)-b (3) Pi coupled excited states is investigated. After resolving, modeling, and interpreting the hyperfine structure of several lines, we successfully identify a long-lived level resulting from the accidental hyperfine coupling between the 0(+) and 0(-) components of the b(3)Pi state, satisfying all the requirements for the population transfer toward the lowest rovibrational level of the X (1) Sigma(+) state. Using two-photon spectroscopy, its binding energy is measured to be 4977.308(3) cm(-1). We calibrate all the transition strengths carefully and also demonstrate Raman transfer of Feshbach molecules to the absolute ground state.