Near-threshold K-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