Ionization of methane and its electronic energy levels

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Abstract
The retarding potential difference (r.p.d.) efficiency curves of methane obtained with two different types of mass spectrometers are presented and discussed. It is shown that a Jahn–Teller splitting could explain some of the experimentally observed energy levels. Pre-ionization is shown to occur at 14.4, 16, and 19.5 eV. In particular, the latter value is rediscussed in terms of the second ionization potential of methane. It is concluded that the recent explanation of Ehrhardt and Linder, according to which the 19.5 eV level is a forbidden pre-ionized state, is compatible with our results and that the second ionization potential of methane should be around 24 eV. Ion–molecule reactions are shown to be of small importance around 19.5 eV, contrary to a hypothesis suggested by Sjögren.

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