The HeH+ ions produced in the He++H2 reaction at Ecm=20 eV are analyzed in coincidence with the other reaction product: either an H*(2s) metastable atom or an H*(2p) atom. The measurements are restricted to laboratory scattering angles of the HeH+ ions in the range 5°<θ<15° encompassing the maximum of the differential cross section for their production. The H*(2s) atoms are observed at a corresponding fixed angle around −90° relative to the incident He+ direction. The Lα radiation which is the signature of the H*(2p) atom is detected at various angles in the collision plane; this provides polarization (in particular alignment) parameters describing the H*(2p) electron cloud. The HeH+ differential cross section maximum is composed of a few peaks associated with the H*(2p or 2s) species. The H*(2s) to H*(2p) cross section ratio is around 0.1. For the HeH+ peaks at small scattering angles, the Lα radiation patterns obtained for the nascent H*(2p) electron cloud are essentially of dipolar type. The measured alignment angle relative to the final HeH+ center of mass direction lies in the range 67°–81°. A simple model is devised to relate these observations to the H*(n=2) probability amplitudes formed right after the reaction, that is, before post-reaction Stark effect in the field of the receding HeH+ ion together with Coriolis coupling modify these amplitudes. The analysis shows that contribution of the 2p⊥ state lying perpendicular to the axis joining the H* atom to the HeH+ center of charge predominates. This is attributed to a dominant role played by the 2pπu crossing between the potential energy surfaces of the (He–H2)+ compound. It also implies, in agreement with earlier predictions of Dhuicq et al. [Chem. Phys. 206, 139 (1996)] that this compound is bent when going through the important {He++H2}→{He+H2+*}→{HeH++H*} stages of the reaction at the investigated energy. At the largest angles investigated, evidence is seen for the emergence of an additional mechanism attributed to the 2sσg crossing.
Vibrational mode and collision energy effects on reaction of H 2 CO + with C 2 H 2 : Charge state competition and the role of Franck-Condon factors in endoergic charge transfer Theoretical study of the reactions of Ar + +H 2 and Ar + +HD using the trajectory surface hopping method
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