Dipole and nondipole photoionization of molecular hydrogen

We describe a theoretical approach to molecular photoionization that includes first-order corrections to the dipole approximation. The theoretical formalism is presented and applied to photoionization of H_2 over the 20- to 180-eV photon energy range. The angle-integrated cross section σ, the electric dipole anisotropy parameter β_e, the molecular alignment anisotropy parameter β_m, and the first-order nondipole asymmetry parameters γ and δ were calculated within the single-channel, static-exchange approximation. The calculated parameters are compared with previous measurements of σ and β_m and the present measurements of β_e and γ+3δ. The dipole and nondipole angular distribution parameters were determined simultaneously using an efficient, multiangle measurement technique. Good overall agreement is observed between the magnitudes and spectral variations of the calculated and measured parameters. The nondipole asymmetries of He 1s and Ne 2p photoelectrons were also measured in the course of this work.