Electron impact ionization of methane

A simple model to calculate the total ionization cross sections of a molecu le due to electron impact has been obtained by combining the useful feature s of the two previous models given by Kim and Rudd (1994 Phys. Rev. A 50 39 54) and Saksena et al (1997 Physica B 233 201, 1997 Int. J. Mass Spectrom. ion Process. 171 L1). It is shown that the Bethe and Mott cross section ter ms obtained by Kim and Rudd are the approximate forms of the present expres sions. The calculated cross sections for the CH4 molecule are in satisfacto ry agreement with a number of experimental data over a wide energy range va rying from 15 eV to 2.7 MeV. Although the present Bethe and Mott cross sect ions in the binary-encounter-Bethe model are different from the correspondi ng cross sections of Kim et al (1997 J. Chem. Phys. 106 1026), the total io nization cross sections obtained in these two investigations are very close to each other. For impact energies greater than about 40 eV the present cr oss sections are in good accord with those given by the Saksena model. Howe ver, at lower energies the present cross sections are greater than those gi ven by the above model and are in better agreement with the experimental da ta. The present value of the collisional parameter is about 10% lower than the experimental value of Reike and Prepejchal (1972 Phys. Rev. A 6 1507).