Electron impact ionization of CH₄: ionization energies and temperature effects

Abstract

The threshold energy and the threshold behaviour for electron impact ionization of CH4 were investigated at two temperatures, 293 and 693 K. The study was performed with a crossed electron–molecule beam apparatus with an electron energy resolution of 120 meV full width at half maximum. The values of the ionization energies (IEs) and the threshold behaviour were determined using a fitting procedure involving a convolution of the cross section and the electron energy distribution function. At 293 K the following IEs were obtained: IE(CH4+/CH 4) = 12.65 ± 0.4 eV, IE1(CH3+/CH 4) = 13.58 ± 0.1 eV (ion pair) and IE2(CH3+/CH 4) = 14.34 ± 0.1 eV. At 693 K a red shift in these IEs of about 0.14 eV was observed for both CH4+/CH 4 and CH3+/CH 4 processes, which reflects the change in the internal energy of CH4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH2+/CH 4) = 15.1 ± 0.1 eV, IE(CH+/CH 4) = 19.8 ± 0.1 eV and IE(C+/CH 4) = 20.5 ± 0.2 eV.