Observation of spin-rotation exchange in ultracold Rb + KRb collisions

Studying atom-molecule collisional physics at ultracold temperature is essential for applications ranging from the sympathetic cooling of molecules to the Feshbach association of polyatomic molecules. Ultracold atom-molecule system also provides a new platform to study the chemistry in a new regime. In a previous work [1], we found that long-lived complexes are formed in Rb + KRb collisions, where Rb atoms are in the ground hyperfine state, and KRb molecules are in the rovibronic ground state. The complexes are photoexcited by the optical dipole trapping light, which explains the rapid atom-molecule loss observed in the samples. We recently found that with Rb prepared in the excited hyperfine state, very few complexes are detected in Rb + KRb collisions. Instead, we discovered that Rb atoms relax to the ground hyperfine state while exciting KRb molecules to rotationally excited states. We directly characterized the atom and molecule states after the collision using resonance-enhanced multi-photon ionization followed by time-of-flight mass spectrometry. Our results reveal a new loss mechanism in atom-molecule collisions and provide a benchmark for future theoretical calculations.

[1] Nichols et al. Phys. Rev. X 12, 011049 (2022)