  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.
