Electron-impact ionization and appearance potentials

Abstract

A method ¹ for the computerized acquisition of ionization efficiency data followed by mathematical analysis using the energy-distribution difference technique,² which is a considerable improvement on conventional mass spectrometric techniques, has been applied to the determination of the ionization and appearance potentials of a variety of compounds and processes. Comparisons of the electronimpact ionization potential data with photoelectron spectroscopic measurements show that it is possible to measure adiabatic ionization potentials by electron impact in favourable cases and indicates a reason for the consistently high values found by other methods. The accuracy of measurement of appearance potentials approaches that of photoionization methods and in one case evidence for a preionization process is found. A high activation energy process is shown to occur without the expected large kinetic shift in appearance potential, suggesting that this computerized method is very sensitive. The measurements also yield a heat of formation and ionization potential for the “phenyl” fragment ion from monosubstituted benzenes.