Feshbach resonances in ultracold K-39

We discover several magnetic Feshbach resonances in collisions of ultracold K-39 atoms, by studying atom losses and molecule formation. Accurate determination of the magnetic-field resonance locations allows us to optimize a quantum collision model for potassium isotopes. We employ the model to predict the magnetic-field dependence of scattering lengths and of near-threshold molecular levels. Our findings will be useful to plan future experiments on ultracold K-39 atoms and molecules.