Collisions of slow electrons with methane: ionisation, fragmentation and resonances

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Abstract
Ionisation efficiency and appearance potential functions for CH4 +, CH3 +, CH2 + and CH+ have been measured using a monoenergetic electron beam colliding with methane. The upper limit for the first ionisation potential of CH4 is determined to be 12.71 eV. Appearance energies for the ionic fragments have yielded values for heats of formation which indicate a large reverse activation energy in the case of CH+. Structure in the ion yield curves for CH4 +, CH3 + and CH2 + in the energy region 19-22 eV is in qualitative agreement with the results of Marmet and Binette (1978) and may be due to highly excited states of methane. The first excited state of CH+ as well as four excited states of CH2 + are observed. Other structure reported in various RPD measurements is not reproduced. The electron transmission function of CH4, measured on two spectrometers over 1-20 eV, reveals a broad minimum at 7.8 eV and a resonance at 18.5 eV.

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