Electron impact ionization energies

Abstract

Ionization energies (IEs) of 25 high temperature effusion-beam species, determined in this laboratory over a period of several decades from electron impact (EI) threshold measurements, are compared primarily with values obtained from photoelectron spectroscopy (PES). In a few instances, the comparisons include values obtained by other spectroscopic methods. The EI values were evaluated by the extrapolated voltage difference method and are assigned an accuracy of 0.10 eV, while the PES values are generally accurate to 0.01–0.02 eV. In the most straightforward cases, where the adiabatic and vertical IEs are coincident, the EI and PES values generally agree to within the lower accuracy of the EI results. Where molecular geometry changes lead to significant differences between adiabatic and vertical IEs, the EI values unexpectedly tend to agree with the adiabatic rather than the vertical IEs. Overall, the comparisons indicate that the Nier-Ingrahm type EI source is capable of reasonably accurate threshold IE measurements, within 0.10 eV, but the interpretation of the results in terms of specific ionic states is, of course, problematic. Keywords: Ionization energies; Electron impact; Photoelectron spectroscopy