Triple Differential Cross Sections for electron-impact ionization of methane at intermediate energy

We report an experimental and theoretical investigation of electron-impact single ionization of the HOMO 1t2 and the NHOMO 2a1 states of CH4 at an incident electron energy of 250 eV. Triple differential cross sections measured in two different laboratories were compared with results calculated within the molecular 3-body distorted wave (M3DW) and generalized Sturmian function (GSF) theoretical models. For ionization of the 1t2 state, the binary peak was observed to have a single maximum near the momentum transfer direction that evolved into a double peak for increasing projectile scattering angles, as has been seen for ionization of atomic p-states. A detailed investigation of this evolution was performed. As expected because of its s-type character, for ionization of the 2a1 state only a single binary peak was observed. Overall, good agreement was found between experiment and theory.