Chemiionization of the hydrogen molecule by He(2 3S) and He(2 1S) atoms: potential surfaces, autoionization widths, and cross sections

Abstract

Potential energy surfaces have been calculated for He(2 3S) and He(2 1S) atoms on H2 using the stabilization method. Energies have been obtained for several He*–H2 separations X and relative orientations ϑ of the H–H and He–H2 axes; the H–H distance was fixed at 1.4 a0. Aside from shallow van der Waals minima at large separations, cuts of both surfaces at fixed ϑ are repulsive. The adiabatic He*–H2 interactions are resonances lying in the HeH+2+e continuum at all points; hence chemiionization can occur in collisions. The autoionization width Γ as a function of nuclear coordinates has been calculated in two ways: (i) by Fermi’s ’’golden rule’’ utilizing expansions in square-integrable functions to avoid having to directly evaluate integrals over continuum orbitals, and (ii) by a new, less rigorous technique based on observations about the convergence characteristics of the stabilization procedure. The widths are found to fall off exponentially with He*–H2 separation and to be only weakly dependent on the angular orientation. The scattering problem has been formulated in terms of the local complex potential V (X,ϑ)−1/2iΓ (X,ϑ) and total chemiionization cross sections have been calculated in the spherical potential and sudden approximations at energies below 10 eV. The two approximations yield very similar results. Ionization rate constants and the singlet-to-triplet cross section ratios are also presented. The cross sections and ratios exhibit a strong dependence on collision energy (or temperature), and it is shown that the well known disagreement between beam and afterglow experiments about these quantities may largely be a result of their different velocity distributions.