We describe the recent progress in the development of the semiempirical approach developed by Jain and Khare for the calculations of ionization cross sections for molecules by electron impact. Along with the state-of-the-art description of this approach, the emphasis will be on the evaluation of cross sections for a carbon dimer, C2, and trimer, C3. Single-differential ionization cross sections as a function of secondary or ejected electron energy and averaged secondary electron energy for C2 and C3 are calculated at fixed impinging electron energies of 100 and 200 eV. The integral ionization cross sections are also derived from ionization threshold to 2000 eV. Extensive comparisons of the presently evaluated direct ionization cross sections with the only available theoretical binary-encounter-dipole model calculations of Kim and Rudd, spherical-complex-optical-potential model calculations of Nagama and Antony, DM model calculations of Deutsch and Märk, and the calculations of Michelin et al. based on the combination of the Schwinger variational iterative method and distorted wave approximation revealed a satisfactory agreement. The present study also establishes the validity of the semiempirical approaches and so provides a solid foundation for further applications to the larger molecules.

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Energy-dependent partial differential ionization cross sections at constant incident electron energies and their integral cross sections for n-butane (n-C4H10) by electron impact corresponding to the formation of various major cations are calculated using a revisited Jain–Khare semiempirical formulation. Averaged energies of the secondary electrons are also derived from the differential cross sections. The ionization rate coefficients as a function of the temperature are evaluated from integral partial ionization cross sections and Maxwell–Boltzmann energy distributions. Calculations of cross sections include the Bethe analytical extrapolation at higher energies up to 6 keV. A satisfactory agreement of the calculated partial and total ionization cross sections with the available experimental and theoretical results is noted.