COLD-COLLISION PROPERTIES DERIVED FROM FREQUENCY-SHIFTS IN A CESIUM FOUNTAIN

We interpret recently measured collisional frequency shifts in a cesiu m atomic fountain using coupled-channel theory and a simple picture of hyperfine-coupled long-range molecular states. We predict the existen ce of Cs2 singlet and triplet states very close to threshold with an a ssociated strong influence on ultracold collisions and large negative values for the Cs + Cs scattering lengths. Our results are important f or future precision experiments using Cs fountains, and may also have important consequences for attempts to realize a Bose condensate in an ultracold atomic Cs vapor.