Theoretical Study On Electron Collision With Excited Helium

Recently, systematic research works on electron-excited atom collisions have been carried out. Studies of electron interaction with excited atoms can offer new opportunities to investigate the electronic structure of atoms, especially the dynamics of electron and atomic collisions. It has attracted a number of experimental and theoretically activities during the past decades. In contrast to corresponding ground state comparisons, there are significant discrepancies for metastable helium between recent experiments and calculations. It is very interesting to note that a long standing discrepancy between the theoretical predictions and experiments exist for the ionization of metastable helium. It reflect the difficulty of giving accurate theoretical description for the dynamic of electron scattering by excited targets.A detailed description of the coupled channels optical method for electron-atom scattering has been given in this paper. In the present calculation, for 2~1S helium, the P-space consists of 10 channels: 2,3,4~1S, 3,4~3S, 2,3~1P, 2,3~3P, 3~1D. The Q-space includes the continuum. The target bound states are represented by configuration interaction(CI) wave functions.We have performed systematic calculations for cross sections of electron scattering by helium in its metastable 21S states to the higher lying excited states. The effects of polarization potential are studied in the CCO model. At low energies, discrepancies among results of different theoretical calculations caused probably by different treatments of the target polarization. Because the lower ionization threshold energy and the higher polarizability of excited helium cause a shift of the ionization cross section to lower energy and an increase in its magnitude. Polarization interaction plays a very important role for scattering cross sections.