Exterior electron model for Penning ionization. Unsaturated hydrocarbons

Abstract

A simple theoretical model is described for Penning ionization (M+A*→M++A+e−) in which a metastable-state rare gas atom (A*) extracts an electron from a target molecule (M) and ejects another electron into a continuum state. One of the most important factors governing Penning ionization is electron distributions of molecular orbitals which are directly connected with the electronic transition probabilities. Another key factor is the repulsive molecular surface which divides the unreactive electron densities in the interior region from the reactive electron densities in the exterior region, because the metastable atom A* cannot penetrate into the interior region. In the present model, exterior electron densities (EED) for individual molecular orbitals are considered to be proportional to Penning ionization branching ratios. Good agreements between ab initio MO calculations of EED’s and experimental branching ratios of Penning ionization were obtained for various unsaturated hydrocarbon molecules.