Molecular collisions in magnetic fields

The control of atom-atom interactions using magnetic fields has been crucial to recent advances in atomic physics. Similar control should be possible for atom-molecule and molecule-molecule collisions. We have generalized the BOUND and MOLSCAT packages to allow calculations in magnetic fields, initially for collisions of molecules in multiplet Sigma states with structureless atoms. We have used the new capability to carry out bound-state and scattering calculations on 3He+NH and 4He+NH as a function of magnetic field. Following the bound-state energies to the point where they cross thresholds gives very precise predictions of the magnetic fields at which zero-energy Feshbach resonances occur. We have located and characterized two very narrow zero-energy Feshbach resonances in 4He+NH. One resonance shows a pole in the scattering length as usually observed for atomic collisions, but for the second resonance the pole in the scattering length is dramatically suppressed and the cross sections show relatively small peaks. The suppression of the pole in the scattering length is due to inelastic scattering. This is a general result, applicable to both atomic and molecular collisions. In general, poles will be strongly suppressed whenever the resonant state is coupled with comparable strength to the elastic and inelastic channels