Electron impact excitation of methane: determination of appearance energies for dissociation products

In this work, we present an experimental study of dissociative excitation of CH4 utilizing a crossed electron molecular beam experiment. Methane was excited by nearly monochromatic electrons generated by a trochoidal electron monochromator. The dissociative products were identified on the basis of the emission spectra in the ultraviolet–visible (UV/VIS) spectral range. The excitation functions were recorded as the function of the electron energy for different emission bands of the fragments (Balmer series for H: n = 3,4…9→2, and moreover, CH: A2Δ→X2Π, CH: B2Σ−→ X2Π, CH: C2Σ+→ X2Π, CH+: B1Δ→ A1Π, and CI: 2p3s 1Po1→2p21S0). From the experimental data we have determined the threshold energies for excitation of particular fragments. Present experimental results indicate that the threshold energies for some dissociative excitation channels could be lower by ∼1–2 eV in comparison to earlier studies and indicate that different dissociative processes may be operative at the threshold than assumed in the former studies. (paper)