Spectrum of Feshbach resonances in NaLi $+$ Na collisions

Collisional resonances of molecules can offer a deeper understanding of interaction potentials and collision complexes, and allow control of chemical reactions. Here, we experimentally map out the spectrum of Feshbach resonances in collisions between ultracold triplet ro-vibrational ground-state NaLi molecules and Na atoms over a range of 1400 G. Preparation of the spin-stretched state puts the system initially into the non-reactive quartet potential. A total of 25 resonances are observed, in agreement with quantum-chemistry calculations using a coupled-channels approach. Although the theory cannot predict the positions of resonances, it can account for several experimental findings and provide unprecedented insight into the nature and couplings of ultracold, strongly interacting complexes. Previous work has addressed only weakly bound complexes. We show that the main coupling mechanism results from spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interaction, and that the collisional complexes which support the resonances have a size of 30-40 $a_0$. This study illustrates the potential of a combined experimental and theoretical approach.