Absolute partial cross sections for electron-impact ionization of CH4 from threshold to 1000 eV

Absolute partial cross sections for the production of CH4+, CH3+, CH2+, CH+, C+, H-2(+), and H+ from electron-impact ionization of CH4 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ions are completely collected. The overall uncertainty in the absolute cross section values is +/-3.5% for singly charged parent ions and is slightly greater for fragment ions. Although previous measurements are generally found to agree well with the present results far CH4+ and CH3+, almost all previous work for the remaining fragment ions lies lower than the present results and in the case of H+ is lower by approximately a factor of 4. (C) 1997 American Institute of Physics.