The triplet s-wave scattering length of
6Li is determined using two-photon photoassociative spectroscopy of the diatomic a
3${\mathrm{\ensuremath{\Sigma}}}_{\mathrm{u}}^{+}$ state of
6Li2. The measured binding energy of the highest-lying bound state, combined with knowledge of the potential, determines the s-wave scattering length to be (-2160\ifmmode\pm\else\textpm\fi{}250)
a0, where
a0 is the Bohr radius. This extraordinarily large scattering length signifies a near-threshold resonance. A complete table of singlet and triplet scattering lengths for collisions involving
6Li and
7Li determined from this and our previous spectroscopic investigations is given.
We report a theoretical study of Feshbach resonances in
6Li+23Na and
7Li+23Na mixtures at ultracold temperatures using accurate interaction potentials in a full quantum coupled-channel calculation. Feshbach resonances for
l=0 in the initial collisional open channel
6Li(f=1∕2,mf=1∕2)+23Na(f=1,mf=1) are found to agree with previous measurements, leading to precise values of the singlet and triplet scattering lengths for the
6,7Li+23Na pairs. We also predict additional Feshbach resonances within experimentally attainable magnetic fields for other collision channels.
We measure a zero crossing in the scattering length of a mixture of the two lowest hyperfine states of
6Li. To locate the zero crossing, we monitor the decrease in temperature and atom number arising from evaporation in a
CO2 laser trap as a function of magnetic field B. The temperature decrease and atom loss are minimized for $B=52.8\ifmmode\pm\else\textpm\fi{}0.4\mathrm{mT},$ consistent with no evaporation. We also present preliminary calculations using potentials that have been constrained by the measured zero crossing and locate a broad Feshbach resonance at $\ensuremath{\approx}86\mathrm{mT},$ in agreement with previous theoretical predictions. In addition, our theoretical model predicts a second and much narrower Feshbach resonance near 55 mT.
We employ radio-frequency spectroscopy on weakly bound (6)Li(2) molecules to precisely determine the molecular binding energies and the energy splittings between molecular states for different magnetic fields. These measurements allow us to extract the interaction parameters of ultracold (6)Li atoms based on a multichannel quantum scattering model. We determine the singlet and triplet scattering lengths to be a(s) = 45.167(8)a(0) and a(t) = -2140(18)a(0) (1a(0) = 0.052 917 7 nm), and the positions of the broad Feshbach resonances in the energetically lowest three s-wave scattering channels to be 83.41(15), 69.04(5), and 81.12(10) mT.
We characterize Feshbach resonances in all isotopologues of the
Li−Li system with improved interaction potentials. Starting from spectroscopically accurate Morse/long-range (MLR) potential-energy curves for the singlet (
X1Σ+) and triplet (
a3Σ+) electronic states of
Li2, 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
6Li−6Li and
7Li−7Li 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
6Li−7Li isotopologue. In its lowest-energy hyperfine channel, all resonances are narrow (
∼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
Li2 molecules in deeply bound rovibrational levels of both the
X1Σ+ and
a3Σ+ potentials across all three isotopologues.
We compare and contrast the wide Feshbach resonances and the corresponding weakly bound states in the lowest scattering channels of ultracold
6Li and
7Li. We use high-precision measurements of binding energies and scattering properties to determine interaction potentials that incorporate non-Born-Oppenheimer terms to account for the failure of mass scaling between
6Li and
7Li. Correction terms are needed for both the singlet and the triplet potential curves. The universal formula relating binding energy to scattering length is not accurate for either system. The
6Li resonance is open-channel-dominated and the van der Waals formula of Gao [J. Phys. B 37, 4273 (2004)] gives accurate results for the binding energies across much of the resonance width. The
7Li resonance, by contrast, is weakly closed-channel-dominated and a coupled-channel treatment of the binding energies is required. Plotting the binding energies in universal van der Waals form helps illustrate subtle differences between the experimental results and different theoretical forms near the resonance pole.