Spin-orbit effects in the photoionization excitation of neon

Using multichannel quantum-defect theoretical methods, we have performed highly correlated R-matrix calculations for the photoionization excitation of neon in the (50--53)-eV photon energy range, where numerous doubly excited resonances are found. By incorporating spin-orbit effects through an intermediate-coupling frame transformation, we have reproduced experimental observance of the significant breakdown of LS coupling in resonant angular distributions and fine-structure partial cross sections. Extreme sensitivity to the target structure in this weak process and strong interactions between neighboring Rydberg series of resonances make a detailed comparison for every resonance difficult, but many of the interesting spin-orbit features, beyond fine-structure splitting of series, are revealed. This method permits a quantitative analysis of LS-forbidden contributions to complex photoionization spectra and should prove useful for studying spin-orbit effects in light atomic systems.