Radio-Frequency Transitions on Weakly-Bound Ultracold Molecules

We show that radio-frequency spectroscopy on weakly-bound molecules is a powerful and sensitive tool to probe molecular energy structure as well as atomic scattering properties. An analytic expression of the rf excitation lineshape is derived, which in general contains a bound-free component and a bound-bound component. In particular, we show that the bound-free process strongly depends on the sign of the scattering length in the outgoing channel and acquires a Fano-type profile near a Feshbach resonance. The derived lineshapes provide an excellent fit to both the numerical calculation and the experimental measurements.