OPTICAL SUPPRESSION OF HYPERFINE-CHANGING COLLISIONS IN A SAMPLE OF ULTRACOLD SODIUM ATOMS

We report the observation of hyperfine-changing collisions and the sup pression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ion ization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppr ess the hyperfine-changing collisions, producing suppression up to 80% . The experimental results are compared with predictions of a simple L andau-Zener model and of a three-dimensional, multichannel close-coupl ing calculation. Both theories agree at low intensity, but at high int ensity the experiment shows a saturation behavior that is qualitativel y described only by the three-dimensional multichannel theory.