Exchange interaction effects in NO core-level photoionization cross-sections

The effect of the exchange interaction in the photoionization continuum is investigated, using N 1s photoionization of NO into the 1s{sup -1}2{pi} ({sup 1}{Pi}) and ({sup 3}{Pi}) final states as an example. The separation in energy of these two final states is 1.41 eV. Significant differences in their partial photoionization cross-sections are observed over a wide range of energies and cannot be accounted for by the different multiplicity of the states. We suggest that the deviation of the {sup 3}{Pi}/{sup 1}{Pi} cross-section ratio from the statistical weighting at intermediate energies is dominated by the difference in the final-state potential experienced by the photoelectron and at asymptotically high energies by the multiplet-dependent amount of intensity going into multi-electron (shake-up) processes. Calculations underpinning this point are presented. We also show supporting measurements of the {sup 3}{Pi}/{sup 1}{Pi} cross-section ratio for O 1s ionization and the absolute photoabsorption cross-section for NO over a wide energy range covering the core level region.