Properties of cold collisions of 39K atoms and of 41K atoms in relation to Bose-Einstein condensation

We have determined properties of cold 39K+39K and 41K+41K collisions by a multichannel inverted perturbation approach applied to spectroscopic data of highly excited 39K singlet and triplet bound states. We predict positive scattering lengths for the 39K+39K and 41K+41K singlet potentials, as well as for the 41K+41K triplet potential, and a negative value for 39K+39K triplet scattering. From a study of the magnetic field dependence we conclude that 41K may be a bosonic atom with a Feshbach resonance in the magnetic field range where these atoms can be trapped in the f=1, mf=-1 hyperfine state, thus providing a possibility to switch between stable and instable condensates in a Bose-Einstein condensation experiment