Low-Energy, Large-Angle Electron-Impact Spectra: Helium, Nitrogen, Ethylene, and Benzene

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Abstract
Electron-impact spectra taken at a fixed scattering angle of 90° and a variety of energies from 10 to 40 eV are reported for helium, nitrogen, ethylene, and benzene. Forbidden singlet—triplet transitions were observed in all the systems studied at all energies. In all cases, the scattering cross sections for various transitions undergo large relative changes near threshold. We find that singlet—triplet transitions become a very important part of the total inelastic scattering cross section below 20 eV. In helium, the spectrum changes radically near threshold. In nitrogen, singlet—triplet transitions dominate the spectrum at energies less than 20 eV. Previous results on the presence of a singlet—triplet transition in ethylene at 4.4 eV have been confirmed. The 6.5-eV ethylene ``mystery band'' transition was not observed at energies at low as 10.9 eV. In benzene, the lowest triplet state is observed near 3.9 eV as well as the three lowest singlet states. The general utility of the very large angle scattering method is discussed.

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