Endoergic chemi-ionization in N–O collisions

Abstract

A semiclassical theory of endoergic chemi-ionization is developed and applied to the ionizing events that occur when ground state oxygen atoms collide with nitrogen atoms in the ground and first excited states. The approach we use is an adaptation and extension of earlier theories due to Bardsley, Nakamura and Miller. For energies in excess of the associative ionization (AI) threshold, EAI≳O, but less than the Penning ionization (PI) threshold, EPI≳EAI, only AI can occur.However, when E≳EPI chemi-ionization may result in the formation of a stable diatomic ion (AI), neutral and ionized atomic fragments (PI), or of a metastable diatomic rotational resonance (DI, delayed ionization). Our theory relates the experimental AI and PI cross sections to the cross sections for these three basic events. The heavy-particle motions are treated classically in terms of adiabatic potential energy functions. However, localized nonadiabatic transitions also are taken into account by using the Landau–Zener approximation. It is assumed that the Franck–Condon principle applies to the electronic transitions between the states of the neutral reactants and the continuum states of the ionic products. Numerical estimates are presented for the AI cross sections of the two reactions N(4S)+O(3P) →NO+(Σ+) + e and N(2D)+O(3P) →NO+(Σ+) + e, for which the threshold energies are 2.76 eV and 0.38 eV, respectively. These calculations are limited to collision energies below 10 eV. They are based on potential energy curves due to Michels and on estimates of the ionization widths that Bardsley and Lee have extracted from spectroscopic information. The theoretical predictions compare well with the results of Ringer and Gentry’s merged beam experiments which have been reported in the preceding paper.