Controlling Cold Collisions of Polar Molecules with External Fields

In this thesis we explore how external fields can be used to control collisions of ultracold polar molecules. First we review the Stark and Zeeman effects for polar molecules and two body multi-channel scattering theory. A general treatment of the Stark effect and dipolar interactions is also presented. We consider cold collisions of OH molecules in the ²Π _3/2 ground state under the influence of a magnetic field. We find that modest fields of several thousand Gauss can act to suppress inelastic collisions of weak-field-seeking states by two orders of magnitude. We attribute this suppression to two factors: (i) an indirect coupling of the entrance and the exit channel, in contrast to the effect of an applied electric field and (ii) the relative shift of the entrance and exit scattering thresholds. In view of these results, magnetic trapping of OH may prove experimentally feasible.

We also present first steps toward understanding the ultracold scattering properties of polar molecules in strong electric field-seeking states. We have found that the elastic cross section displays a quasi-regular set of potential resonances as a function of the electric field, which potentially offers intimate details about the intermolecular interaction. We illustrate these resonances using a "toy" model composed of pure dipoles and a more physically realistic system. To analyze these resonances, we use a simple WKB approximation to the eigenphase, which proves both reasonably accurate and meaningful.