Formation of ultracold weakly bound dimers of bosonic (NaK)-Na-23-K-39

We create weakly bound bosonic (NaK)-Na-23-K-39 molecules in a mixture of ultracold Na-23 and K-39. The creation is done in the vicinity of a so far undetected Feshbach resonance at about 196 G which we identify in this work by atom-loss spectroscopy. We investigate the involved molecular state by performing destructive radio-frequency binding-energy measurements. For the constructive molecule creation we use radio-frequency pulses with which we assemble up to 6000 molecules. We analyze the molecule creation efficiency as a function of the radio-frequency pulse duration and the atom number ratio between Na-23 and K-39. We find an overall optimal efficiency of 6% referring to the K-39 atom number. The measured lifetime of the molecules in the bath of trapped atoms is about 0.3 ms.