ELECTRON-IMPACT DISSOCIATION OF METHANE INTO CH3 AND CH2 RADICALS .1. RELATIVE CROSS-SECTIONS

The abundances of primary neutral radicals CH3 and CH2 resulting from electron-impact dissociation of methane have been measured. This was accomplished with threshold-ionization mass spectrometry in a differentially pumped dual-electron-beam device. The relative partial cross sections for dissociation into CH3 and CH2 are reported for electron energies between 10 and 100 eV. A maximum cross section is found at 20-30 eV for both CH3 and CH2 radicals. At higher energies, the cross section decreases rather sharply, in particular for dissociation into the CH2 radical.