Structure in the angular dependence of optical suppression of photoassociative ionization collisions within an optically cooled and collimated sodium atom beam

We report measurements of the angular distribution of optical suppression in photoassociative ionization collisions taking place within a bright, slow Na atom beam. At a collision "temperature" (T = E-kin/k(B)) Of about 5 mK we observe rich structure in the suppression measure as a function of the angle between the atom-beam axis and the polarization axis of the suppressor light beam. This structure cannot be explained by the superposition of individual partial-wave contributions to the suppression measure, and we propose a partial-wave interference phenomenon to account for it.