Angle-energy distributions of Penning ions in crossed molecular beams. I. Evidence for discrete nonadiabaticity in the He*(2 ¹S)+H, D→He+H⁺,D⁺+e^- reaction

Relative doubly differential cross sections for the Penning ionization of H and D by He* (2 ¹S) are reported at a mean collision energy of 9.2 kcal/mol for H and 7.9 for D in an experiment employing crossed supersonic beams and a rotatable mass spectrometer detector. The products are scattered sharply forward in the center-of-mass frame, with release of an additional 3.5 kcal/mol into translational energy at the peak of the recoil distribution. The energy release is significantly lower than predicted on the basis of quantum dynamics on a single adiabatic ab initio complex potential, or of the experimental low-energy Penning electron spectrum (PIES). Discrete nonadiabatic behavior is suggested and a model is developed based on modifications of ab initio potentials, Penning ionization atom-atom theory on a single complex potential, and an adiabatic-velocity modification of Landau-Zener curve-crossing theory (which may be more generally useful). The dynamical model compares well with exact two-state complex potential calculations for the magnitude and energy dependence of the total ionizatipn cross section, and improves the emulation of both the low-energy PIES data and our recoil distributions. Significant modifications to the ab initio potentials include a deepening of the well in the He* (2 ¹S) + H adiabatic potential by 2.0 to 10.2±1 kcal/mol, a quadrupling of the magnitude of the resonance width, and a reduction in the adiabatic splitting at the avoided crossing between 2 ¹S and 2 ³P potentials. Our low-energy calculations suggest that available data on the associative ionization (HeH⁺) branching fraction is inconsistent with the PIES data.