Electron impact ionization energies

Ionization energies (IEs) of 25 high temperature effusion-beam species, det ermined in this laboratory over a period of several decades from electron i mpact (EI) threshold measurements, are compared primarily with values obtai ned from photoelectron spectroscopy (PES). In a few instances, the comparis ons include values obtained by other spectroscopic methods. The EI values w ere evaluated by the extrapolated voltage difference method and are assigne d 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 vert ical Ifs are coincident, the EI and PES values generally agree to within th e lower accuracy of the EI results. Where molecular geometry changes lead t o significant differences between adiabatic and vertical Ifs, 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. (C) 2000 Elsevier Science B.V.