Theoretical determination of absolute electron-impact ionization cross sections of molecules

Much emphasis has been devoted recently to the experimental determination of absolute electron-impact ionization cross sections of molecules and radicals because of the importance of these cross sections in many applications (e.g. as input parameters to modeling codes for various purposes). Supporting theoretical calculations have been lagging behind to some extent. Because of the inherent complexity of such calculations, simplistic additivity rules and semiempirical methods have often been used in place of more rigorous calculation schemes, particularly in applications where a larger number of cross section data were needed with reasonable precision. Although these methods have often proved to be quite successful as descriptive tools (i.e. reproducing existing experimental ionization cross sections reasonably well), their ability to calculate cross sections for species for which no experimental data are available (predictive capabilities) tend to be limited or questionable. This topical review describes recent progress in the development of more rigorous approaches for the calculation of absolute electron-impact molecular ionization cross sections. The main emphasis will be on the application of the semiclassical Deutsch-Märk (DM) formalism, which was originally developed for the calculation of atomic ionization cross sections, to molecular targets, and on the binary-encounter-Bethe (BEB) method of Kim and Rudd. The latter is a simpler version of the more rigorous binary-encounter-dipole (BED) theory, which was also first developed for the calculation of atomic ionization cross sections, and based on the methods developed by Khare and co-workers. Extensive comparisons between available experimental cross sections and the predictions of these theoretical models will be made for 31 molecules and free radicals (H₂, N₂, O₂, S₂, C₂, C₃, O₃, H₂O, NH₃, CO₂, CH₄, CH₃, CH₂, CH, CF₄, CF₃, CF₂, CF, NF₃, NF₂, NF, SiF₃, SiF₂, SiF, TiCl₄, C₂H₂, C₂H₆, C₆H₆, SiF₆, C₂F₆, CH₃OH).