Rotational Population Measurement of Ultracold ^85Rb^133Cs Molecules in the Lowest Vibrational Ground State

We measure the rotational populations of ultracold ^85Rb^133Cs molecules in the lowest vibrational ground state by a depletion spectroscopy and quantify the molecular production rate based on the measurement of single ion signal area. The ^85Rb^133Cs molecules in the X^1\it \Sigma^+ (v=0) are formed from the short-range (2)^3\it \Pi_0^+(v=10,J=0) molecular state. A home-made external-cavity diode laser is used as the depletion laser to measure the rotational populations of the formed molecules. Based on the determination of single ion signal, the production rates of molecules in the J=0 and J=2 rotational levels are derived to be 4800 mole/s and 7200 mole/s, respectively. The resolution and quantification of molecules in rotational states are facilitative for the manipulation of rotational quantum state of ultracold molecules.