Low-Energy Electron Scattering by Complex Atoms: Theory and Calculations

Abstract

This chapter reviews theoretical methods for the quantitative description of electron scattering by atoms with more than one electron, in the low-energy region. Electron scattering by helium provides an excellent comparison between theory and experiment, because many experimental data are available, and relatively accurate theoretical calculations are feasible. The quantitative theory of electron scattering by a complex atom requires consistent treatment of the N-electron target atom and of the (N + 1)-electron scattering system. For a neutral target atom, low-energy scattering is dominated by the long-range polarization potential due to dynamical distortion of the atom by the incident electron. At short range, the external electron becomes part of a transient negative ion, whose specific states produce resonance structures in scattering cross sections. Much of the observed energy-dependent structure in electron-atom scattering arises from such resonances. Energy can be transferred between the incident electron and target atom, inducing transitions from the initial atomic state. As the energy of the incident electron is increased, successively higher excited states of the atom become energetically accessible. Characteristic scattering structures can occur at each excitation threshold. At low energies, only a relatively small number of partial waves (incident electron angular momentum states) contribute significantly to electron scattering. This number increases with incident electron energy and with the strength of the dominant long-range interaction potential.