Diatomic molecular-orbital correlation diagrams for Penning and associative ionization

Abstract

The collision processes ${A}^{*}+B\ensuremath{\rightarrow}A+{B}^{+}+{e}^{\ensuremath{-}}$ (Penning ionization) and ${A}^{*}+B\ensuremath{\rightarrow}A{B}^{+}+{e}^{\ensuremath{-}}$ (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 ${\mathrm{He}}^{*}(1s2s)+\mathrm{Ar}(3{p}^{6})$ within the multiple-scattering $X\ensuremath{\alpha}$ 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 ${\mathrm{He}}^{*}(1s2s)+\mathrm{Ar}(3{p}^{6})$, $+\mathrm{Kr}(4{p}^{6})$, $+\mathrm{Hg}(6{s}^{2})$, and ${\mathrm{Ne}}^{*}(2{p}^{5}3s)+\mathrm{Ar}(3{p}^{6})$, 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.