The CH4 + electroionisation efficiency curve and the CH3 +, CH2 +, CH+ and C+ appearance potential curves are presented. The data were obtained by an apparatus of very high sensitivity with an improved electron monochromator. The CH4 + ionisation potential is measured to be 12.63+or-0.02 eV. The results show that some previously reported appearance potential values should be corrected, in some cases by several electron volts. This is attributed to superior apparatus sensitivity. Ion-pair formation is clearly observed for the CH3 + and CH+ fragments. The CH+ and C+ appearance potentials have been detected down to adiabatic threshold. The stable fragment CH2 2+, is also observed.

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A source of monoenergetic electrons giving a high electron current (3×10−8 A) simultaneously with a good energy resolution (30 meV) is described. It has been ascertained that the emitted electron beam is not superimposed on a background which normally appears when an electron beam is scattered on deflecting electrodes within the instrument. This double 127° cylindrical selector is especially suitable for experiments where relatively high intensities in conjunction with high quality beams are essential and where stable electron currents over periods of several days or weeks are required. Such a stability is observed in the case where the gas pressure inside the selectors is as high as 10−5 Torr.

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