Cold collisions in an atomic beam

We report a measurement of the rate constant for associative ionization (AI) between excited Na(3p) atoms colliding within a single beam at a velocity of 11.9 msec^-1, corresponding to a temperature of 65 mK. The collisions take place in a narrow velocity class of excited atoms produced by crossing the collimated Na beam with a monomode dye laser at an angle of 45^∘{}. Light polarized linearly in the collision plane yields an AI rate constant greater than that for circular polarization by a factor of 2. We find K_65mKˡⁱⁿ =(12.2±{}4.3)×{}10^-13 cm³ sec^-1 and K_65mKᶜⁱʳ =(6.1±{}1.8)×{}10^-13 cm³ sec^-1. The experimental arrangement also permits absolute calibration of earlier crossed-beam measurements of the excitation function (cross section versus velocity) at conventional thermal beam kinetic energies. Present results together with earlier work show that the cross-sectional temperature dependence is in qualitative accord with theory.