Collision energy dependence of product branching in Penning ionization: He*(2 1S, 2 3S) + H2, D2, and HD

Relative ionization cross sections for the title systems with articulation of all product ion channels have been measured in the collision energy range 1.5–4.0 kcal/mol using crossed supersonic molecular beams; the H2 results have been extended down to 0.5 kcal/mol by the use of a 10% H2/Ar seeded beam. The data are interpreted with a microscopic two-step model that assumes ionization near the turning point in the excited state, a centrifugal barrier criterion for ionic complex formation, and statistical partitioning of flux among the possible ionic products, i.e., phase-space theory. A full statistical calculation underestimates the amount of rearrangement ionization He*+H2→HeH++H+e− by a factor of 2, but one which excludes antiparallel coupling of orbital and rotational angular momenta in the H+2 channel is in better accord with the data. A substantial isotope effect favoring HeD+ over HeH+ in the HD reactions by a factor of 1.9±0.2 is well represented by the model.