Triple differential cross-section measurements for electron-impact ionization of methane from a coplanar geometry to the perpendicular plane.

A new study of electron-impact single ionization of the HOMO 1t2 state of CH4 has been conducted at incident electron energies 20 eV and 40 eV above the ionization energy of the state. Triple differential cross sections were measured from a coplanar symmetric geometry, where scattered and ionized electrons were detected at equal angles, through to the perpendicular geometry where the outgoing electrons emerged orthogonal to the incident electron beam. At the lower energy, the electrons were detected with equal energies of 10 eV, whereas at the higher energy, data were obtained for equal energies of 20 eV and for unequal energies of 5 eV and 35 eV. The results are compared to a molecular 3-body distorted wave approximation that used a full averaging procedure to allow for the random orientation of the target, an orientation averaged molecular orbital model that averages the target wavefunction over all orientations prior to the collision, and a distorted wave Born approximation that does not include postcollisional interactions.

Low energy (e,2e) measurements of CH4 and neon in the perpendicular plane.

Low energy experimental and theoretical triple differential cross sections for the highest occupied molecular orbital of methane (1t(2)) and for the 2p atomic orbital of neon are presented and compared. These targets are iso-electronic, each containing 10 electrons and the chosen orbital within each target has p-electron character. Observation of the differences and similarities of the cross sections for these two species hence gives insight into the different scattering mechanisms occurring for atomic and molecular targets. The experiments used perpendicular, symmetric kinematics with outgoing electron energies between 1.5 eV and 30 eV for CH(4) and 2.5 eV and 25 eV for neon. The experimental data from these targets are compared with theoretical predictions using a distorted-wave Born approximation. Reasonably good agreement is seen between the experiment and theory for neon while mixed results are observed for CH(4). This is most likely due to approximations of the target orientation made within the model.