Characterization of Feshbach resonances in $^6\mathrm{Li}{-}^7\mathrm{Li}$ using improved interaction potentials

TL;DR

This paper improves interaction potentials for lithium isotopologues, accurately characterizes Feshbach resonances, and predicts narrow, strongly closed-channel resonances in $^6$Li-$^7$Li, aiding ultracold molecule formation.

Contribution

It develops refined interaction potentials and provides detailed characterization of Feshbach resonances in all lithium isotopologues, including the heteronuclear $^6$Li-$^7$Li system.

Findings

All resonances are narrow (~0.01-0.1 G) in the lowest hyperfine channel.

Resonances are strongly closed-channel dominated and mainly triplet in character.

Results enable design of pathways for ultracold Li$_2$ molecule production.

Abstract

We characterize Feshbach resonances in all isotopologues of the $\mathrm{Li}{-}\mathrm{Li}$ system with improved interaction potentials. Starting from spectroscopically accurate Morse/long-range (MLR) potential-energy curves for the singlet ($X^{1}\Sigma^{+}$) and triplet ($a^{3}\Sigma^{+}$) electronic states of $\mathrm{Li}_2$, we apply small phenomenological inner-wall adjustments (following Julienne and Hutson, Phys. Rev. A 89, 052715 (2014), arXiv:1404.2623v3) and fit the resulting potentials to threshold measurements for the $^{6}\mathrm{Li}{-}^{6}\mathrm{Li}$ and $^{7}\mathrm{Li}{-}^{7}\mathrm{Li}$ isotopologues, including binding energies, scattering lengths, and Feshbach resonance positions. Using the optimized potentials in coupled-channels scattering calculations, we predict and characterize s-wave Feshbach resonances in the $^{6}\mathrm{Li}{-}^{7}\mathrm{Li}$ isotopologue. In its lowest-energy hyperfine channel, all resonances are narrow ($\sim 0.01{-}0.1$ G), strongly closed-channel dominated, and predominantly triplet in electronic spin character, in marked contrast to the homonuclear systems. These results provide a foundation for designing Raman optical-transfer pathways to produce ultracold $\mathrm{Li}_2$ molecules in deeply bound rovibrational levels of both the $X^1\Sigma^{+}$ and $a^3\Sigma^{+}$ potentials across all three isotopologues.