Dissociation of CH4 and CD4 by electron impact: Production of metastable and high-Rydberg hydrogen and carbon fragments

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Abstract
The production of hydrogen and carbon atoms in metastable and high‐lying Rydberg states due to electron‐impact dissociation of CH4 and CD4 has been investigated for incident electron energies from threshold to 300 eV. Onset energies for the production of metastable hydrogen atoms were observed at electron impact energies of 22.0±0.5, 25.5±0.6, 36.7±0.6, and 66±3 eV, and at 26.6±0.6 eV for the production of metastable carbon atoms. Most of the excited H fragments appear to have been formed initially in high‐lying Rydberg states with the metastable H(2S) state populated principally by cascade. The differential cross section, (dσ/dΩ)90°, for the dissociative excitation of H atoms to metastable and high‐lying Rydberg states reached a maximum value of approximately 1×10−19 cm2/sr at 100 eV. At the same energy, the metastable carbon differential cross section is 2×10−20 cm2/sr.