Paths to ultracold ground-state Li_2 — contrasting Feshbach resonances in different isotopologues

Ultracold molecules have emerged as a powerful platform for controlled chemistry [1]. At T < 1 mK, all quantum numbers of the reactants—including the collisional partial wave—are well defined, providing a clean starting point for reaction dynamics studies. Yet the complexity of the heavy systems studied so far hampers detailed theoretical understading and limits the scope of control over the reactivity. Motivated by the field-defining insights gained from simple reactions of light species (H + HD → H₂ + D; H₂ + F → HF + H) [2], we plan to investigate isotope-exchange reactions of lithium in the ultracold regime. A prerequisite is the formation of Li₂ isotopologues (⁶Li₂, ⁷Li₂, ⁶Li⁷Li) in their absolute ground states via coherent association of pairs of ultracold Li atoms. Here we present theoretical studies of magneto-association of weakly bound Li₂ via Feshbach resonances (“Feshbach molecules”). Despite their chemical similarity, the isotopologues have distinct threshold physics, leading to markedly different resonances and Feshbach-molecules. We discuss the impacts of Feshbach molecule properties on the subsequent Raman optical transfer (STIRAP) to the absolute ground state. Finally, we discuss the potential of our system for the coherent control of bimolecular reactions.

[1] Liu, Y. & Ni, K.-K. Bimolecular Chemistry in the Ultracold Regime. Annu. Rev. Phys. Chem. 73, (2022).

[2] Yang, X. State-to-State Dynamics of Elementary Bimolecular Reactions. Annu. Rev. Phys. Chem. 58, 433–459 (2007).