Formation of HeH+ from Low-Energy Collisions of Metastable Helium and Molecular Hydrogen

Abstract

A merging-beams technique has been used to study HeH+ formation resulting from the interaction of a composite beam of He(21S) and He (23S) with a beam of H2. The experiments were conducted over a range of interaction energy W from 0.05 to 10 eV. It was learned that the dominant process for producing HeH+ is the rearrangement ionization reaction He*+H2→HeH++H+e, where He* represents the metastable helium. The experiment consisted of measuring the lab-energy distributions of HeH+. Relative and absolute cross sections were obtained from these distributions. The distributions indicate that, in the center-of-mass system, most of the HeH+ is scattered in the direction of the incident He*. The energies associated with the HeH+ at the peaks of these distributions are very close to those expected for spectator stripping. Conservation of energy for the reaction can only be achieved by assuming that the electron carries away a substantial portion of the available energy. It is shown that the two-step model He*+H2→He+H2+†+e→HeH++H+e (where † indicates ground-state or vibrationally excited H2+) satisfactorily explains the measured lab-energy distributions near W=0.05 eV but does not agree completely with the results for W≥ 2 eV.