Very-high-precision bound-state spectroscopy near a ${}^{85}\mathrm{Rb}$ Feshbach resonance

We precisely measured the binding energy $\left({\epsilon }_{\mathrm{bind}}\right)$ of a molecular state near the Feshbach resonance in a ${}^{85}\mathrm{Rb}$ Bose-Einstein condensate (BEC). Rapid magnetic-field pulses induced coherent atom-molecule oscillations in the BEC. We measured the oscillation frequency as a function of B field and fit the data to a coupled-channel model. Our analysis constrained the Feshbach resonance position [155.041(18) G], width [10.71(2) G], and background scattering length $\left[-{443\left(3\right)a}_0\right]$ and yielded new values for the Rb interaction parameters. These results improved our estimate for the stability condition of an attractive BEC. We also found evidence for a mean-field shift to ${\epsilon }_{\mathrm{bind}}.$