A Feshbach resonance in collisions between ultracold ground state molecules

TL;DR

This paper reports the first observation of a narrow Feshbach resonance in collisions between ultracold ground state NaLi molecules, demonstrating long-lived complexes and opening avenues for coherent control of chemical reactions.

Contribution

It provides the first experimental evidence of a Feshbach resonance in ground state molecules, showing that such resonances can exist despite high density of states and rapid decay.

Findings

Narrow (25 mG) Feshbach resonance observed in NaLi molecules

Resonance enhances collisional loss rate by over two orders of magnitude

Resonance occurs at nearly degenerate open channels, indicating complex decay pathways

Abstract

Collisional resonances are an important tool which has been used to modify interactions in ultracold gases, for realizing novel Hamiltonians in quantum simulations, for creating molecules from atomic gases and for controlling chemical reactions. So far, such resonances have been observed for atom-atom collisions, atom-molecule collisions and collisions between Feshbach molecules which are very weakly bound. Whether such resonances exist for ultracold ground state molecules has been debated due to the possibly high density of states and/or rapid decay of the resonant complex. Here we report a very pronounced and narrow (25 mG) Feshbach resonance in collisions between two ground state NaLi molecules. This molecular Feshbach resonance has two special characteristics. First, the collisional loss rate is enhanced by more than two orders of magnitude above the background loss rate which is saturated at the $p$-wave universal value, due to strong chemical reactivity. Second, the resonance is located at a magnetic field where two open channels become nearly degenerate. This implies the intermediate complex predominantly decays to the second open channel. We describe the resonant loss feature using a model with coupled modes which is analogous to a Fabry-P\'erot cavity. Our observations prove the existence of long-lived coherent intermediate complexes even in systems without reaction barriers and open up the possibility of coherent control of chemical reactions.