Near-thresholdK-shell absorption cross section of argon: Relaxation and correlation effects

A detailed theoretical study has been made of the effects of core relaxation and electron-electron correlation on the near-threshold absorption cross section of argon, and the results are compared with previous calculations and experiment. The key results are the following: (1) Within the Hartree-Fock approximation, core relaxation must be included to obtain realistic cross sections; (2) final-state calculations are extremely sensitive to the way in which exchange effects are included in the calculations; (3) although electron-electron correlations produce only small changes in the single-electron cross section, they can account for the two-electron resonances which have been observed immediately above the ionization threshold. In particular, the main resonance above threshold appears to be 1s3p⁵3d², which can be reached from the ground state via initial-state correlation.