OBSERVATION OF A FESHBACH RESONANCE IN COLD ATOM SCATTERING

We probe s-wave collisions of laser-cooled Rb-85(f = 2, m(f) = -2) ato ms with Zeeman-resolved photoassociation spectroscopy. We observe that these collisions exhibit a magnetically tunable Feshbach resonance, a nd determine that this resonance tunes to zero energy at a magnetic fi eld of 163 +/- 7 G. This result indicates that the self-interaction en ergy of an Rb-85 Bose-Einstein condensate can be magnetically tuned. W e also demonstrate that Zeeman-resolved photoassociation spectroscopy provides a useful new tool for the study of ultracold atomic collision s.