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 highest occupied molecular orbital 1t₂ and the next highest occupied molecular orbital 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 and generalized Sturmian function 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.