Ultracold collisions in magnetic fields: reducing inelastic cross sections near Feshbach resonances

) in magnetic fields, with the NH molecule in its n = 1 rotationally excited states. We haveexplored the pattern of levels as a function of magnetic field and identified the nearly good quantumnumbers in different regimes. We have used the bound-state calculations to locate low-energyFeshbach resonances. When the magnetic field is used to tune across such a resonance, the realand imaginary part of the scattering length show asymmetric oscillations and peaks with amplitudebetween 1 and 3 ˚A. The scattering length does not pass through a pole at resonance. The resonantbehavior is characterized by a complex resonant scattering length a