Hyperfine-resolved optical spectroscopy of ultracold ⁸⁷Rb¹³³Cs molecules: the b ³Π₀ metastable state

Using an ultracold gas of ⁸⁷Rb¹³³Cs molecules, we perform hyperfine-resolved spectroscopy of transitions from the vibronic ground state to the lowest rovibrational states of the electronic state b ³Π₀, as a function of magnetic field. These transitions are spin forbidden, resulting in narrow linewidths, and feature near-diagonal Franck-Condon factors. We develop a model of the hyperfine and Zeeman structure that includes coupling between the 0^+ and 0^- components of b ³Π₀. We fit the spectra to obtain rotational and hyperfine coupling constants. We measure transition dipole moments associated with specific transitions by directly observing Rabi oscillations as a function of a resonant laser pulse duration. Using resonant π pulses, we prepare molecules in the electronically excited state and directly measure the spontaneous emission rate.