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 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 +/- 0.003 cm(-1). We also report on the two-color pulsed-laser ionization used to locate the depletion transitions.