Radio-frequency Association of Weakly-bound 23Na-40K Molecules in an Interacting Thermal Mixture

The production of weakly bound molecules via a magnetic Feshbach resonance is a prerequisite step in the creation of ground state molecules from ultracold atomic mixtures using the two-photon association technique. This intermediate step is typically achieved by an adiabatic ramp across the Feshbach resonance or, as we explore in this work, with a free-to-bound radio-frequency association process. Our rf spectra reveal the existence of additional short-lived bound states whose binding energies closely track that of the well-known diatomic Feshbach molecule state. We are able to explain these binding energies from a universal theory involving only the interatomic Van der Waals lengths and s-wave scattering lengths.