Collisional Studies of NaCs Molecules

Obtaining a detailed understanding of loss processes in ultracold molecular gases is a key frontier of the field and a direct link to quantum chemistry. So far, experimental findings have been ambiguous. Some experiments show strong evidence that molecules are lost when long-lived four-body collisional complexes are excited by trap light, while other experiments do not. Newly available ultracold NaCs molecules offer a valuable data point in this fast-evolving field. First, we report on an efficient STIRAP pathway to the molecular ground state. Next, we study the collisional properties of NaCs molecules. Using a modulated dipole trap with dark times of up to 1.5 ms, we do not see evidence for the excitation of complexes via trapping light. Further enhancing the detection sensitivity, we have implemented an approach to study NaCs molecules in the absence of trapping light for longer periods of time. These measurements exclude (NaCs)₂ complex lifetimes below 100 ms. Finally, we measure the temperature dependence of the two-body loss rate of NaCs molecules and observe loss rates larger than the universal relation. We thus speculate that the RRKM theory of collisional complex lifetimes may not apply to the case of NaCs.