which determines the strength of the Coulomb interaction between the ejected photoelectron and the remaining molecule. In photoionization of atoms, Z = 1 is used to account for the interaction between the electron and the positively charged ionized core, while in photoionization of molecules, an effective partial charge Zeff may be used instead. The Coulomb wave treatment of 𝜓 has been used with success to calculate photoionization cross sections in small and mediumsized molecules, but the accurate description of photoelectron angular distributions remains problematic with this approach. This is demonstrated again here for SO  in Fig. , where the computed curves do not agree with the experimental data. With Coulomb waves, the contribution of spherical partial waves with ℓ 0 becomes large near the ionization threshold, causing the value of β to increase above 0 even at near-threshold energies.