Rotationally resolved depletion spectroscopy of ultracold KRb molecules

We report on the use of depletion spectroscopy to detect ultracold ground-state KRb molecules with rotational resolution. The population of a specific ground-state vibrational level v^″ , produced by photoassociation of ultracold atoms, is monitored by one-color two-photon pulsed-laser ionization. When a cw laser is resonant with a rovibrational transition to an excited state, the ground-state population, and hence the ion signal, is depleted. This narrow-band spectroscopic technique allows the individual rotational levels in both ground and excited states to be resolved, and thus the population of a single ground-state rovibrational level to be monitored. These resolved transitions are a necessary first step in transferring population from high vibrational levels of the ground state, such as produced by photoassociation, to the absolute molecular ground state XΣ+1 ( v^″=0 , J^″=0 ). This technique can also be used to determine binding energies of high- v^″ molecules. Combining our binding energy measurement with the results of previous spectroscopy yields an improved dissociation energy of the X state: D_e=4217.822±0.003cm^-1 . We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.