Angular and energy analysis of HeH+ products from the charge exchange excitation reaction

The angular and energy distributions of the HeH+ products resulting from He+ + H2 collisions have been measured in the center-of-mass energy range 9 eV < Ecm < 34 eV. These measurements confirm those performed previously by us at Ecm = 20 eV and lead themselves to an investigation of the reaction mechanism. The HeH+ molecule is formed in its first ν ≤ 3 vibrational levels while the hydrogen atom is found essentially in the H∗ (n = 2) state. At all energies the angular distribution is strongly peaked; the peak maximum moves from the cm angle χ ≈ 55° to χ ≈ 30° when the energy increases within the above range. The results are related to two-electron transitions occurring at diabatic potential surface crossings of the (HeH2)+ transient molecule. Accordingly a theoretical framework involving a distorted wave — infinite order sudden approach is devised to rationalize the observations. This leads to a “generalized reflection principle” which, when submitted to a stationary phase treatment, leads to a description of the reaction mechanism in terms of two mere elastic half-collisions. The model which is arrived at explains the general trends of the measured vibrational and angular distributions.