Dipole and nondipole photoelectron angular distributions of molecular hydrogen*

Molecular hydrogen has long been a prototype system for experimental and theoretical studies of photoionization. Measurements of angle-integrated cross sections and the anisotropy parameter $\beta $ provide experimental tests of molecular photoionization theory and calculational approaches within the dipole approximation. Nondipole interactions distort dipole angular-distribution patterns and can be probed by measurements of forward-backward asymmetries with respect to the photon propagation vector. Nondipole asymmetries can be calculated to first order in theoretical treatments that include cross terms between electric-dipole and electric-quadrupole or magnetic-dipole photoionization amplitudes. In this work we report measurements of the dipole anisotropy parameters $\beta $ and the nondipole asymmetries $\gamma $+3$\delta $ of H$_{2}$ over the 20--150 eV photon-energy range. Comparison is made with calculations based on first-order corrections to the dipole approximation with amplitudes calculated within the single-channel, static-exchange approximation.