Ionization of hydrogen and deuterium atoms in thermal energy collisions with metastable He*(23S) atoms

We report experimental and theoretical results for the basic autoionization processes He*(23S) + H/D(12S), leading to Penning (→ He + H+/D+ + e- (Θ, ε)) and associative ionization (→ HeH+/HeD+ (ν+, J+) + e- (Θ, ε)). The experimental electron energy spectra, measured at thermal collision energies (E-barrel ≅ 50 meV), show an angular-dependent behaviour which is due to the interplay between the orientation of the collision complex at the moment of autoionization - as referred to the relative velocity direction - and a strongly non-isotopic internal angular distribution for electron emission from the excited quasi-molecule. The ab initio work on He*(23S) + H/D includes electron angular momentum dependent complex (resonance-to-continuum) coupling elements, revealing substantial contributions of autoionization into the electronic d wave. Comparison between the angle-differential experimental electron spectra, measured with an energy resolution of 30 meV, and the corresponding theoretical distributions, obtained in the framework of the local complex potential theory, yields good agreement for both isotopes. (author)