  Energy transfer between different mechanical degrees of freedom in
atom-molecule collisions has been widely studied and largely understood.
However, systems involving spins remain less explored, especially with a
state-to-state precision. Here, we directly observed the energy transfer from
atomic hyperfine to molecular rotation in the $^{87}$Rb ($|F_a,M_{F_a}\rangle =
|2,2\rangle$) + $^{40}$K$^{87}$Rb (in the rovibronic ground state $N=0$)
$\longrightarrow$ Rb ($ |1,1\rangle$) + KRb ($N=0,1,2$) exothermic collision.
We probed the quantum states of the collision products using resonance-enhanced
multi-photon ionization followed by time-of-flight mass spectrometry. We also
carried out state-of-the-art quantum scattering calculations, which rigorously
take into account the coupling between the spin and rotational degrees of
freedom at short range, and assume that the KRb monomer can be treated as a
rigid rotor moving on 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, suggesting that
vibrational degrees of freedom and conical intersections play an important part
in ultracold Rb + KRb collisions. Additionally, our ab initio calculations
indicate that spin-rotation coupling is dramatically enhanced near a conical
intersection, which is energetically accessible at short range. The
observations confirm that spin is coupled to mechanical rotation at short range
and establish a benchmark for future theoretical studies.
