Abstract
We have studied the formation of Na-3(+) ions in collisions of Na(3d, 5s, 4d) atoms with vibrationally excited Na-2(upsilon ") molecules in their electronic ground state in a supersonic beam. The cross-section of the Na(4d(5/2)) + Na-2 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(Sd) with Na-2(upsilon ") is observed only with vibrationally excited Na-2 molecules. The cross-section sigma(upsilon ") of this reaction increases only slightly with upsilon " for low vibrational excitation, while a strong increase with upsilon " is observed for upsilon " greater than or equal to 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 upsilon " greater than or equal to 26.