Photoionization Cross Sections of He and H₂

We combine accurate measurements at low energies and theoretical calculations at high energies to construct sets of photoionization cross sections of He and H-2 that should be reliable at all energies. We control the accuracy of the recommended cross section data by ensuring that they satisfy a range of oscillator strength sum rules. We provide accurate analytic formulae for the photoionization cross sections that have the proper asymptotic form. At high frequencies the cross sections for H-2 are 2.8 times those for H.