Cooper minima: a window on nondipole photoionization at low energy

Photoionization of Mg 3s is studied near the Cooper minimum in dipole channels using the relativistic-random-phase approximation. While the importance of first-order nondipole effects on photoelectron angular distributions at low energies is well known, it is reported here for the first time that in the energy region near the dipole Cooper minimum, quadrupole transitions are not just important, but actually dominate the total photoionization cross section. Studies of dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interference terms in the photoelectron angular distribution show that in the region of dipole Cooper minimum even the calculation of the dipole angular distribution parameter, β, requires the inclusion of quadrupole channels. The significance of second-order [O(k2r2)] nondipole terms, primarily due to the contributions from electric quadrupole-quadrupole interference terms at photon energy as low as ∼11 eV, are shown to induce dramatic changes in the photoelectron angular distribution over a small energy range. (fast track communication)