Velocity-selected atomic-beam collisions in the energy range from 300 to 5 K

We report cross-section measurements of the associative-ionization process, Na(3${\mathit{p}}^2$ ${\mathit{P}}_{3/2}$)+Na(3${\mathit{p}}^2$ ${\mathit{P}}_{3/2}$)→${\mathrm{Na}}_2^{+}$+e as a function of velocity, alignment, and orientation in the collision energy range from 300 to 5 K. Two independently tunable laser beams select narrow velocity classes of excited sodium in a well-collimated atomic beam. Circular and linear polarization of the excitation lasers permits alignment and orientation of the excited atoms in the two velocity classes. The excitation functions show pronounced differences reflecting different molecular pathways to the final product, and they bridge the energy range between conventional and ‘‘cold’’ collision dynamics.