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

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⁺, 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.