Results from a single-excitation C1 calculation, whilch is comparable to the IVO calculation, got results of 10.55 and 10.96eV for the 3 T 2 and 1 T 2 thresholds respectively and 0.572 for the 1 T 2 oscillator strength. Numerical stability proved to ve a critical issue in these studies. At higher energies, the uncorrected 1 T 2 cross section drops off rapidly, while the dipole contribution becomes quite substantial. The scattering is fairly isotropic near threshold and becomes forward peaked at higher energy. The shapes of the experimental and theoretical cross sections are similatr at intermediate angles but the theoretical result is of larger magnitude.
Several factors complicate the study of electron impact exciation cross sections in methane.
These include the dissociation character of the excited states, the close spacing of thresholds, and the fact that vertical exciations reach strongly repulsive parts of the upper potential energy surfaces. Therefore it's difficult to determine the origin of observed defferences between theoretical results and the limited experimental data acailable. Other limitations of present calculations may also affect the comparison with experiment. The dipole scattering contribution to the singlet cross section is sensitive to the oscillator strength of the excitation as well. Despite the limitations, the present calculation is useful in elucidating the potential sensitiviey of the cross sections the Rydberg excitations to coupling among excited states and does provide a reasonable estimate of the CH 2 fragment cross section.