OBSERVATION OF A SHAPE RESONANCE IN THE COLLISION OF 2 COLD RB-87 ATOMS

We observe a shape resonance in the scattering of two ultracold Rb-87 atoms, causing the colliding atoms to form a long-living compound syst em inside an l=2 centrifugal barrier. Its existence follows from a pho toassociation experiment in a gas sample of doubly polarized Rb-87 ato ms. Using it we are able to carry out direct determinations of the tri plet scattering length for Rb-87, relevant to Bose-Einstein condensati on experiments, and of the Rb+Rb C-6 dispersion coefficient. Consequen ces for the Rb-85 scattering length are discussed.