Differences in the Angular Dependencies of Spin- and Symmetry-Forbidden Excitation Cross Sections by Low-Energy Electron Impact Spectroscopy

Optically forbidden electronic transitions can be produced by low-energy electron impact. Recent experimental investigations of helium (1-3) have shown that the differential scattering cross sections for forbidden excitations are generally enhanced relative to those for allowed ones at low incident energies and large scattering angles. have now observed marked differences in the angular and energy dependencies of differential cross sections for various kinds of forbidden (spin, symmetry, or both) transitions in helium at low incident energies. Such differences may well provide a basis for determining the nature of optically forbidden transitions detected by electron-impact spectroscopy in other atoms and molecules.