IONIZATION CROSS-SECTIONS OF MOLECULES DUE TO ELECTRON-IMPACT

Recently, Mayol and Salvat have employed a semi-phenomenological appro ximation along with the first Born approximation to calculate K-shell ionization cross-sections of atoms. Their method requires optical osci llator strengths (OOS) as input instead of the generalised oscillator strengths (GOS). As the generation of the GOS for molecules is a diffi cult task, in the present paper we have extended the semi-phenomenolog ical approximation of Mayol and Salvat to investigate molecular ionisa tion because of the availability of OOS for a large number of molecule s. Cross-sections for the total ionisation and for the formation of th e parent ions of H-2, N-2, O-2, NH3, H2O and CO2 by electron impact ar e calculated in a modified first Born approximation for the incident e nergy E varying from the threshold of ionisation to 1 keV. The contrib ution of the exchange is included through the Ochkur approximation. Di ssociative ionisation cross-sections of NH3, H2O and CO2 giving rise t o NH2+ OH+ and O+ ions, respectively, have also been evaluated, The ca lculated cross-sections are, in general, in good agreement with the ex perimental data over a wide energy range (E greater than or equal to 5 0 eV) and the same holds for ratios of the partial to the total ionisa tion cross-sections.