DETERMINATION OF TOTAL CROSS SECTIONS FOR THRESHOLD EXCITATION AND IONIZATION BY ELECTRON IMPACT

Total cross sections have been determined for electronic excitation and ionization processes in helium, carbon monoxide, nitrogen, oxygen, carbon dioxide, ethylene and benzene. The formation of transient negative ions greatly influences the behaviour of the cross sections as a function of the incident electron energy. In our apparatus we can determine the production of low-energy ions as well as the production of low-energy electrons resulting from an inelastic collision. Therefore, cross sections for excitation processes can be normalized on the well-known ionization cross sections. Assuming a value determined by Rapp and Englander-Golden1) for the formation of positive ions from carbon monoxide as a standard, we find for the maximum in the cross section of the 23S excitation of helium 4 × 10-18 cm2 which is reached at about half a volt above threshold. Maier-Leibnitz2) determined 5 × 10-18 cm2 for this maximum. Our results for the threshold excitation of molecules can not be compared with other measurements because the latter are not yet available. The maximum cross section for the formation of negative ions from O2 near 6.5 eV is found to be 1.4 × 10-18 cm2. Rapp and Briglia3) determined the same value for this process. © 1969.