Electron impact ionization of CH4: ionization energies and temperature effects

The threshold energy and the threshold behaviour for electron impact ionization of CH 4 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(CH 4 + /CH 4 ) = 12.65 ± 0.4 eV, IE 1 (CH 3 + /CH 4 ) = 13.58 ± 0.1 eV (ion pair) and IE 2 (CH 3 + /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 CH 4 + /CH 4 and CH 3 + /CH 4 processes, which reflects the change in the internal energy of CH 4 with increasing temperature. In addition, at 293 K IEs were determined also for the small fragment ions, i.e. IE(CH 2 + /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.