Anisotropy of optical suppression in photoassociative ionization collisions within a slow, collimated sodium atom beam

We report the observation of polarization anisotropy in the optical suppression of photoassociative ionization (PAI) collisions. Binary collisions occur within a tightly collimated and optically cooled velocity group of a single sodium beam. A laser beam, blue-detuned with respect to the atomic resonance transition, crosses the atomic beam and suppresses the PAI rate. We measure this suppression as a function of intensity and electric field vector aligned parallel or perpendicular to the atomic beam axis. Quantum close-coupling and multichannel curve-crossing calculations of model systems yield insight into the nature of the anisotropy and are in reasonable agreement with the measurements