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 $X{}^1\Sigma ^{+}$ ($v^{″}=0$, $J^{″}=0$). 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: $D_e=4217.822\pm 0.003{\mathrm{cm}}^{-1}$. We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.

• Received 30 December 2006

DOI:https://doi.org/10.1103/PhysRevA.75.032511