Collisional ionization of Na, K, and Cs by CO2, COS, and CS2: molecular electron affinities

The negative ion products resulting from collisions between orthogonal beams of alkali metal atoms (Na, K, Cs) and the linear triatomic molecules CO2, COS, and CS2 have been studied from threshold to approx. 400 eV (lab). Ions with masses corresponding to the parent molecules CO2, COS, and CS2 are detected for all collision permutations except for Na colliding with CO2. The following electron affinities are deduced from measurements of the threshold for the ion pair production reactions: CO2(-0.60 +- 0.2 eV), COS(+0.46 +- 0.2 eV), and CS2(1.0 +- 0.2 eV). The CO2- ion was found to be metastable with respect to autodetachment. This result is compatible with the negative electron affinity for CO2 and in agreement with our earlier observations of CO2-* and with recent theoretical calculations. The lifetime of CO2-* (9 +- 2 x 10-5 sec) was measured to be independent of collision energy over the region of energy studied (threshold to approx. 20 eV c.m.). The fragment ions O-/CO2, O-/COS, S-/COS, and S-/CS2 were detected at a threshold energy which is consistent with the known energetics of the reactions. These results are compared with recent dissociative electron attachment studies