Dissociative excitation of CH4 by electron impact: Emission cross sections for the fragment species Original Research Article

Absolute emission cross sections for the hydrogen Lyman series, Balmer series CH(A2Δ → X2Π: λ = 420–440 nm), C(2p3s → 2p2: λ = 165.7 nm) and C(2s2p3 → 2s22p2: λ = 156.1 nm) produced by electron impact on CH4 have been measured in the energy range from the apperance threshold to 1000 eV. Since all these appearance potentials rise above the first ionization potential (∼ 12.6 eV) of CH4, super-excited states play important roles in producing excited fragment species. In the low energy region, a few appearance potentials are observed for these fragments, and dissociation channels are discussed. Fano-plot analysis shows that the excited hydrogen atoms are mainly produced via electronic states that have forbidden transitions to the CH4 ground state.