Low-Energy Scattering of Electrons by Helium

The close-coupling equations for electron-helium scattering have been solved in the energy range near the $n=2$ thresholds. Cross sections for elastic scattering from both ground and excited states, for excitation of the ground state to the $n=2$ states (2³S, 2¹S, 2³P, and 2¹P), and for excitation and de-excitation processes involving only the $n=2$ levels are presented and compared with experimental evidence on total metastable production, on angular distributions of excitation cross sections to the $n=2$ levels, and on processes involving only the $n=2$ states. The percentage polarization of light emitted by electron-impact excitation to the 2³P and 2¹P states is computed and compared with experiment. The calculations indicate the importance of resonances in near threshold excitation and de-excitation processes in He. An attempt has been made to understand the resonant structure by considering both the energy dependence of the eigenphases of the many-channel S matrix produced by solving the close-coupling equations and the energy dependence of eigenvalues of the related time-delay matrix.