The He++H2→HeH++H*(2s,2p) reaction: Study of the s and p contributions in a coincidence-correlation experiment

Abstract

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.

Observation of D2H+ (or H2D+) products in coincidence with excited H* (or D*) atoms in the reaction of D2+ with H2

“…Each channeltron can detect radiation of wavelengths Ͻ1500 Å, with a maximum at Ϸ650 Å. 8 A MgF 2 window can be inserted between the second grid and the channeltron in each part of the detector to prevent all particles but photons of wavelength Ϸ(L␣)ϭ1216 Å to reach the channeltron so that the signal-to-noise ratio may be improved. Actually such windows have been used only in the preparation of the measurements presented hereafter and to check some of them when possible.…”

“…For r 1 ϭr x , the system follows this promoted curve with an average probability of 50% ͑as compared with a nearly 100% probability for the He ϩ ϩH 2 (r e ) system͒. [6][7][8] In the same way as for process ͑1͒ it can thus reach curve crossings leading to charge transferexcitation, excitation, and plausibly to the products of reaction ͑2͒.…”

“…At first glance we see that the main features of these maps are rather different from those obtained previously in the He ϩ ϩH 2 case. 8 In particular the peaks are not localized in a single (u,) region, especially in the case of direct photons. The values for which the intensity is maximum are generally smaller than those found for HeH ϩ in Ref.…”

“…One of the reasons is of course the small values of T , due themselves in particular to small values of the intermolecular distances where PES crossings could arise, at high energy. [6][7][8] We have recently performed one of such studies in the E cm ϭ9 -34 eV range. 7 It concerned the reaction…”

“…A coincidence study of the products showed a predominance of the 2 p state of H* over the 2s one. 8 Considering now the collision of H 2 ϩ with H 2 , in certain conditions ͑to be specified below͒ the H 2 ϩ projectile may also bring an inner vacancy in the system as the He ϩ ion does in reaction ͑1͒. A similar excitation mechanism is then plausible.…”