Reaction Dynamics in Negative Ions (S-, O-, CN-) on H2 Collisions

Abstract:
A dynamical study of the various channels in some X-/H2 (X=S,O,CN) collisional systems is performed in a crossed-beam experiment with multicoincidence detection of two products.  This experiment allows the simultaneous analysis of all competing channels (ionic channel and various detachment channels).

The experiment consists in crossing a fast ionic beam with a supersonic beam of H2 at right angle. The fast neutrals (X,XH,XH2) and slow ionic products (e-, H-) are independently detected by two position sensitive detectors and time and position correlated to provide the whole spacial and energetic distribution of the products. The laboratory angles and velocities are converted into center of mass variables to obtain differential cross-sections in the center-of-mass system. The results are presented as velocity vector diagram in the center-of-mass frame.

In the 3 systems X-/H2 (X=S,O,CN), the experimental dynamical behavior of the competition between the various channels is well accounted for by considering the adiabatic or diabatic behavior at the crossing seam existing between two ionic diabatic surfaces X-HH and XHH-. The main conclusion is that, at low collision energy, the 3 channels: reactive charge transfer (R), simple detachment (SD) and reactive detachment (RD) result from an adiabatic behavior at the ionic crossing seam.

In the S-/H2 and O-/H2 systems, a symmetric forward-backward angular distribution is observed at low collision energy in the 3 channels R,RD and SD, indicating that the intermediate complex is an ionic complex (XHH-). A model of an orbiting intermediate is proposed. The lifetime of the complex is calculated.

The study of the CN-/H2 system was performed in order to evidence 2 approaches leading to the 2 isomers HCN and CNH. In this regard, the results were not clear due to the existence of new degrees of freedom in a 4-atom system and no definitive conclusion can be given.
Dr. Kamal Goudjil 
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