Photon- and electron-impact ionization and ejected-electron angular distributions from molecules including retardation effects: Nonrelativistic theory

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Abstract
The nonrelativistic first-Born-approximation matrix element for the photoionization and electron impact of a single electron from a molecule with no restriction on its shape, including retardation effects and with rotational energy resolution, is evaluated within the framework of the Born-Oppenheimer approximation. Angular distributions of ejected electrons are cylindrically symmetric about the propagation direction of the photon or electron source if the light is circularly polarized or if no attempt is made to simultaneously measure the scattered and ejected electrons in electron-impact ionization. If plane polarized light is used, retardation effects destroy the cylindrical symmetry about the light propagation direction. If the scattered electron in electron-impact ionization is detected in coincidence with the ejected electron then the angular distribution of ejected electrons shows cylindrical symmetry about the momentum-transfer direction. Use of an LCAO-MO (linear combination of atomic orbitals representation of molecular orbitals) basis for both bound and continuum one-electron orbitals leads to selection and propensity rules for rotational excitation.