Direct measurement of the velocity dependence of the associative-ionization cross section in Na(3p)+Na(3p) collisions

We present a direct measurement of the velocity dependence of the associative-ionization (AI) cross section for collisions between Na${(}^3$P) atoms in the energy range 0.08–0.29 eV in a crossed-beam experiment using a Doppler-shift velocity-selection technique. The AI cross section at first decreases, reaches a minimum at about 0.15 eV, then sharply increases with collision energy. Interpretation of the results in terms of a semiclassical theory of AI indicates that more than one potential surface of Na${(}^2$P${+\mathrm{}}^2$P) must be involved in the AI process.