ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
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. © 1998 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.476588
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