Electric-octupole and pure-electric-quadrupole effects in soft-X-ray photoemission

Second-order [O(k(2)), k = omega/c] nondipole effects in soft-x-ray photoem ission are demonstrated via an experimental and a theoretical study of angu lar distributions of neon valence photoelectrons in the 100-1200 eV photon- energy range. A newly derived theoretical expression for nondipolar angular distributions characterizes the second-order effects using four new parame ters with primary contributions from pure-quadrupole and octupole-dipole in terference terms;Independent-particle calculations of these parameters acco unt for a significant portion of the existing discrepancy between experimen t and theory for Ne 2p first-order nondipole parameters.