The evolution of inner-shell multielectronic X-ray spectra from threshold to saturation for low- to high-Z atoms

Inter- and intra-shell electronic correlations in an atom are manifested in, and can be studied through, the X-ray emission spectra of multielectronic transitions. We have measured the spectra of K^hα_1,2 hypersatellites (HS), Kα_3,4 satellites (S), and the 3d electron shake-up satellites (D) accompanying the Kα_1,2 diagram lines, and their evolution from the double-ionization threshold, for selected 4- and 5-row elements of the periodic table. These spectra originate in the [1 s]^-1, [2p]^-1, and [3d]^-1 spectator two-electron transitions [1 s]^-2→[1s2p]^-1, [1s2p]^-1→[2p]^-2, and [1s3d]^-1→[2p3d]^-1 respectively. Photoexcitation by monochromatized synchrotron radiation together with high-resolution crystal spectrometers were employed. Ab initio relativistic multiconfigurational Dirac-Fock (RMCDF) calculations reproduce the spectra very well for all transitions, with the HS spectra showing a strong dependence on QED effects. The excitation thresholds were determined accurately. The threshold-to-saturation energy range of the intensity was found to depend strongly on the principal quantum number n of the spectator hole. For example, for Cu, ranges of 2%, 10%, and 60% of the threshold energy are found for the D, S and HS spectra, respectively. Only the D spectrum's evolution conforms to the Thomas model, indicating a dominant shake up/off process. The S and HS spectra seem to be dominated by the knockout ("two-step-one") process near threshold. The S spectra show a two-regime behavior. Near threshold both shape and intensity vary with excitation energy, and above that only intensity changes are observed. The lower-than-expected K^hα₁/K^hα₂ intensity ratio found for Z=23-30 and 39-46 indicates that the angular momenta coupling scheme at the regime intermediate between the LS coupling at low-Z and the jj coupling at high-Z may not be fully accounted for by the prevailing theory.