Associative ionization in collisions of electronically excited Na atoms with vibrationally excited Na₂(v″) molecules

We have studied the formation of Na⁺₃ ions in collisions of Na(3d, 5s, 4d) atoms with vibrationally excited Na₂(v″) molecules in their electronic ground state in a supersonic beam. The cross-section of the Na(4d_5/2) + Na₂ associative ionization is found to be about six times larger than that of the corresponding atomic reaction Na(4d_5/2) + Na(3s) → Na⁺₂. Associative ionization in collisions of Na(3d) with Na₂(v″) is observed only with vibrationally excited Na₂ molecules. The cross-section σ(v″) of this reaction increases only slightly with v″ for low vibrational excitation, while a strong increase with v″ is observed for v″ ≥ 26. The latter observation suggests the opening of the channel leading to the formation of Na⁺₃ ions in the first electronically excited state for v″ ≥ 26.