An ab Initio Molecular Orbital and Dynamics Study on Penning Ionization of Ar with He Metastables(He*(2S,23S)) into the Spin−Orbit Ar+(2P3/2,P1/2) States

Abstract

The Penning ionization Ar + He*(21S,23S) → Ar+(2P3/2,2P1/2) + He + e- is investigated with ab initio and chemical reaction dynamics calculations. Ab initio molecular orbital calculations have been carried out to obtain resonance potential energy curves and partial widths for each spin−orbit ionized state. The result is compared with optical potentials obtained from experiments. The total widths are not well described by single-exponential functions both for the singlet and triplet entrance channels. This indicates that a more flexible functional form should be used to obtain the imaginary part of an optical potential. The total ionization cross sections are calculated by quasiclassical trajectory as well as quantum dynamics in the collision energy range of 0.05−1 eV. The calculations show that the ratio of the cross section for the 2P3/2 ionized state to that for the 2P1/2 state increases with collision energy, in agreement with experimental results. It is found that the sum of the partial ionization cross sections significantly deviates from the total ionization cross section at higher collision energies in quantum dynamics calculations. One should be cautious for the application of quantum dynamics treatment to treat partial ionization in particular at higher energies. Differential cross sections are also calculated by a quantum-dynamics scheme.