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A new method is described which permits ionization and appearance potentials to be determined reproducibly, within ±0.02 volt, by electron impact in a conventional 180° mass spectrometer. The energy spread of the ionizing electrons, while not eliminated, is accounted for in an analytical expression that describes the ionization efficiency curve of argon near the initial upward break. It is shown that an ionization or appearance potential can be determined graphically by drawing a tangent of critical slope to the efficiency curve which has been plotted on a semilog scale. Absolute values are obtained by referring the unknown to the ionization potential of a calibrating gas (usually krypton). For the monatomic gases studied, electron impact and spectroscopic values agree within the experimental uncertainty. For some known polyatomic molecules, the ``vertical'' impact values exceed the ``adiabatic'' spectroscopic potentials by 0.02 to 0.2 volt, depending on molecular complexity. Paraffins, 1-olefins, and 2-olefins show a systematic reduction in potential with chain length. A similar decrease is observed when the mono-olefins are arranged according to the number and size of substitutions on the basic ethylene structure.
Richard E. Honig (1948) studied this question.