Double K-shell photoionization and hypersatellite x-ray transitions of 12 ≤ Z ≤ 23 atoms

Single-photon double K-shell ionization of low-Z neutral atoms in the range 12 <= Z <= 23 is investigated. The experimental method was based on measurements of the high-resolution K alpha(h) hypersatellite x-ray spectra following the radiative decay of the K-shell double-vacancy states excited by monochromatic synchrotron radiation. The photon energy dependence of the double K-shell ionization was measured over a wide range of photon energies from threshold up to and beyond the maximum of the double-to-single photoionization cross section ratios. From the high-resolution x-ray emission spectra the energies and linewidths of the hypersatellite transitions, as well as the K alpha(h)(1):K alpha(h)(2) intensity ratios, were determined. The relative importance of the initial-state and final-state electron-electron interactions to the K-shell double photoionization is addressed. Physical mechanisms and scaling laws of the K-shell double photoionization are examined. A semiempirical universal scaling of the double-photoionization cross sections with the effective nuclear charge for neutral atoms in the range 2 <= Z <= 47 is established.