Observation of a Feshbach Resonance in Cold Atom Scattering

We probe $s$-wave collisions of laser-cooled ${}^{85}\mathrm{Rb}({f=2,m}_f=-2\mathrm{})$ atoms with Zeeman-resolved photoassociation spectroscopy. We observe that these collisions exhibit a magnetically tunable Feshbach resonance, and determine that this resonance tunes to zero energy at a magnetic field of $164\pm 7\mathrm{G}$. This result indicates that the self-interaction energy of an ${}^{85}\mathrm{Rb}$ Bose-Einstein condensate can be magnetically tuned. We also demonstrate that Zeeman-resolved photoassociation spectroscopy provides a useful new tool for the study of ultracold atomic collisions.