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 ${\mathrm{msec}}^{\mathrm{-}1}$, 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 $K_{65\mathrm{m}\mathrm{K}{}^{\mathrm{lin}}}$ =(12.2±4.3)×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^3$ ${\sec }^{\mathrm{-}1}$ and $K_{65\mathrm{m}\mathrm{K}{}^{\mathrm{cir}}}$ =(6.1±1.8)×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^3$ ${\sec }^{\mathrm{-}1}$. 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.