Multipole effects in the angular distribution of photoelectrons

The influence of contributions corresponding to different multipoles L of a radiation field on the angular distribution of photoelectrons upon photoionization of an atom is considered for linearly polarized photons. Relativistic calculations of the differential photoionization cross section are carried out taking into account all multipoles, whose contributions are appreciable, for all shells of atoms with Z = 10, 30, 54, 80, and 94 in the region of photoelectron kinetic energies 0.5 keV ≤ Ek ≤ 500 keV. The self-consistent Dirac-Fock-Slater potential is used. The calculated results showed that for photoelectron energies Ek < or approx. 5 keV, the angular distribution is, on the average, well described in the quadrupole approximation (L ≤ 2). However, the quadrupole approximation is insufficient for the adequate description of the experimentally measured nondipole parameter ζ. In this case, the next, octupole approximation (L = 3) contributes significantly. For energies Ek (greater-or-similar sign) 5 keV, it is necessary to consider higher multipoles (up to L ≅ 50 at Ek = 500 keV). As the energy increases, the angular distribution of photoelectrons becomes increasingly asymmetric and is shifted toward smaller angles