A simple model to calculate the total ionization cross sections of a molecule due to electron impact has been obtained by combining the useful features of the two previous models given by Kim and Rudd (1994 Phys. Rev. A 50 3954) 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 terms obtained by Kim and Rudd are the approximate forms of the present expressions. The calculated cross sections for the CH4 molecule are in satisfactory agreement with a number of experimental data over a wide energy range varying from 15 eV to 2.7 MeV. Although the present Bethe and Mott cross sections in the binary-encounter-Bethe model are different from the corresponding cross sections of Kim et al (1997 J. Chem. Phys. 106 1026), the total ionization cross sections obtained in these two investigations are very close to each other. For impact energies greater than about 40 eV the present cross sections are in good accord with those given by the Saksena model. However, at lower energies the present cross sections are greater than those given by the above model and are in better agreement with the experimental data. 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).

Differential, integrated elastic and momentum transfer cross sections have been calculated for the scattering of electrons by SO 2 , CS 2 and OCS molecules in the energy range 100-1000 eV and total cross sections (elastic + absorption) over a wide incident energy range 100-4000 eV. A relatively simple approach, namely the independent atom model (IAM) with partial waves has been employed. In the present investigation static, polarization, exchange and absorption effects are taken into account. The present results agree fairly well with the experimental data and other theoretical calculations wherever available.

Khare et al observed that in the binary-encounter Bethe approximation their ionization cross sections were very close to those obtained by the method of Kim and Rudd. An explanation has been advanced for the above observation.

Electron impact ionization cross sections (EIICS) of K-shell have been evaluated for light atoms (C, N, O) at incident energies ranging from ionization threshold to 1 GeV. The plane wave Born approximation is used in the proposed model by incorporating it in exchange, coulomb and relativistic effects along with the contributions of transverse interaction to ionization cross sections. In present model, we require two constants for each atom, ionization energy (I ) and the electron occupation number (N ). Adequate comparisons have been made with other theoretical methods, empirical formulae. The predicted EIICS of K-shell also compared with experimental data. Obtained results are in good agreements with available experimental data.

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