Controlling chemistry with magnetic fields: Feshbach resonances for NaLi molecules

NaLi molecules have special properties among ultracold molecules. Due to their small van der Waals length, cross sections for inelastic collisions are small, although the molecule is very reactive. A longstanding goal of chemistry at the quantum level is the control of chemical reactions with external fields. This has motivated the search for Feshbach resonances in molecular systems. However, it is possible that many systems don’t have any resonances due to short lifetimes of collisional complexes or high density of states, preventing resonances from being resolved. So far, only a single molecule-molecule resonance has been observed, in collisions between triplet NaLi molecules. For atom-molecule collisions, resonances have been seen only for NaLi + Na and NaK + K. I will report on our studies of Feshbach resonances with NaLi. A rich spectrum of 25 resonances in spin-stretched NaLi + Na collisions could be explained using state-of-the-art quantum-chemistry calculations. They involve collisional complexes of size 30 – 40 Bohr radii and are due to spin-rotation and spin-spin couplings in combination with the anisotropic atom-molecule interactions. In contrast, our observations of inelastic collisions between NaLi molecules are not well understood and imply that even reactive molecules without reaction barrier can support long-lived collisional complexes.