Observation of a shape resonance in the collision of two cold 87Rb atoms

We observe a shape resonance in the scattering of two ultracold 87Rb atoms, causing the colliding atoms to form a long-living compound system inside an l=2 centrifugal barrier. Its existence follows from a photoassociation experiment in a gas sample of doubly polarized 87Rb atoms. Using it we are able to carry out direct determinations of the triplet scattering length for 87Rb, relevant to Bose-Einstein condensation experiments, and of the Rb+Rb C6 dispersion coefficient. Consequences for the 85Rb scattering length are discussed