Resonant magnetic field control of elastic scattering of cold 85Rb
  A magnetic field dependent Feshbach resonance has been observed in the elastic scattering collision rate between atoms in the F = 2, M = -2 state of 85 Rb. Changing the magnetic field by several Gauss caused the collision rate to vary by a factor of 10,000, and the sign of the scattering length could be reserved. The resonance peak is at 155.2(4) G and its width is 11.6(5) G. From these results we extract much improved values for the three quantities that characterize the interaction porential: the van der Waals coefficient C6, the singlet scattering length, and the triplet scattering length. 
J. L. Roberts, N. R. Claussen, James P. Burke, Jr., Chris H. Greene,   E. A. Cornell, and C. E. Wieman