Nondipolar Asymmetries of Photoelectron Angular Distributions

We have measured the nondipolar contribution to the Ar $1s$ photoelectron angular distribution over the 30--2000 eV electron-energy range. The nondipolar interaction results in a forward or backward asymmetry with respect to the photon beam. The asymmetry is directed backward near threshold, is symmetric near 230 eV, and becomes increasingly forward directed at higher energies. The measured asymmetries are in excellent agreement with theoretical calculations, which include interference between the electric-dipole and electric-quadrupole photoionization amplitudes.