Ultracold photoassociative ionization collisions in an atomic beam: Optical field intensity and polarization dependence of the rate constant

We report here measurements of two-body photoassociative ionization collisions between sodium atoms within an ultranarrow velocity class selected from a well-collimated, thermal atomic beam. Doppler-shifted excitation of the Na(3[ital s] [sup 2][ital S][sub 1/2];[ital F]=1)[r arrow]Na(5[ital p] [sup 2][ital P][sub 3/2]) transition at 285 nm by a single-mode laser defines the narrow velocity class that subsequently populates the [ital F]=2 hyperfine level of the Na ground state by optical pumping. Probe laser beam excitation tuned near the Na(3[ital s] [sup 2][ital S][sub 1/2];[ital F]=2)[r arrow]Na(3[ital p] [sup 2][ital P][sub 3/2];[ital F]=3) transition produced photoassociative ionization with an average collision energy [ital E][sub [ital K]] corresponding to a temperature of 5.3 mK ([ital E][sub [ital K]]=3/2[ital kT]). We determine the rate constant for the process as a function of probe beam intensity and polarization.