Hyperfine-to-rotational energy transfer in ultracold atom-molecule collisions of Rb and KRb

Energy transfer between different mechanical degrees of freedom in atom-molecule collisions has been studied and largely understood. However, systems involving spins remain less explored. In this study, we directly observed energy transfer from atomic hyperfine to molecular rotation in the ⁸⁷Rb ( $$ ) + ⁴⁰K⁸⁷Rb (X¹Σ⁺, rotational state N = 0) ⟶ Rb ( $$ ) + KRb (N = 0, 1, 2) collision with state-to-state precision. We also performed quantum scattering calculations that rigorously included the coupling between spin and rotational degrees of freedom at short range under the assumption of rigid-rotor KRb monomers moving along a single potential energy surface. The calculated product rotational state distribution deviates from the observations even after extensive tuning of the atom-molecule potential energy surface. In addition, our ab initio calculations indicate that spin-rotation coupling is enhanced close to a conical intersection that is energetically accessible at short range. This, together with the deviation, suggests that vibrational degrees of freedom and conical intersections play an important part in the coupling. Our observations confirm that spin is coupled to mechanical rotation at short range and establish a benchmark for future theoretical studies.