Diatomic molecular-orbital correlation diagrams for Penning and associative ionization

The collision processes $A^{*}+B\to A+B^{+}+e^{-}$ (Penning ionization) and $A^{*}+B\to A{B}^{+}+e^{-}$ (associative ionization), where $A$ and $B$ are atoms, are considered in terms of the molecular-orbital (MO) correlation diagrams associated with the reactant and product channels. MO correlation diagrams are calculated for ${\mathrm{He}}^{*}\mathrm{}\left(1s2s\right)+\mathrm{Ar}\left(3\mathrm{}{p}^6\right)$ within the multiple-scattering $X\alpha$ one-electron self-consistent-field scheme for both spin-polarized and non-spin-polarized orbitals. The ionization process is discussed in terms of an Auger-type mechanism. By inspecting the MO's involved in the Auger-type process in their united-atoms limit, an analysis is made of the angular momentum contributions needed in describing the emitted electron in the body-fixed frame. A procedure is proposed for constructing MO correlation diagrams based on atomic-orbital energies at the separated- and united-atoms limits, which are determined from readily available data on ground-state energies of neutral atoms. Estimated MO correlation diagrams are presented for the four collision pairs ${\mathrm{He}}^{*}\mathrm{}\left(1s2s\right)+\mathrm{Ar}\left(3\mathrm{}{p}^6\right)$, $+\mathrm{Kr}\left(4\mathrm{}{p}^6\right)$, $+\mathrm{Hg}\left(6\mathrm{}{s}^2\right)$, and ${\mathrm{Ne}}^{*}\left(2\mathrm{}{p}^{5}3s\right)+\mathrm{Ar}\left(3\mathrm{}{p}^6\right)$, and in each case an analysis is made of the angular momentum components of the emitted electron in the body-fixed frame. The results confirm that relatively few $l$ values need be included in describing the emitted electron. Our analyses show that it is important to use spin-polarized MO's to obtain detailed behaviors at pseudocrossings, and that the effect of Born-Oppenheimer rotational coupling must be considered, particularly between MO's that converge to the same united-atoms limit.