ULTRACOLD PHOTOASSOCIATIVE IONIZATION COLLISIONS IN A MAGNETOOPTICAL TRAP - THE OPTICAL-FIELD-INTENSITY DEPENDENCE IN A RADIATIVELY DISSIPATIVE ENVIRONMENT

We report here measurements of two-body associative-ionization collisi ons between sodium atoms confined in a magneto-optic trap. These colli sions represent a kind of ''open'' or dissipative collision for which the energy of the atom plus applied light field subsystem need not be conserved due to spontaneous-emission coupling to the vacuum modes of the radiation field. The experiment measures the photoassociative-ioni zation-rate constant as a function of the optical field intensity from about 40 to 260 mW cm-2. These results are in reasonable agreement wi th the predictions of an optical-Bloch-equation theory [Y. B. Band and P. S. Julienne, Phys. Rev. A 46, 330 (1992)], but differ strongly fro m the predictions of a local-equilibrium theory (A. Gallagher, Phys. R ev. A 44, 4249 (1991)].