Role of L -shell single and double core-hole production and decay in m -fold ( 1 $\leqslant$ m $\leqslant$ 6 ) photoionization of the Ar$^+$ ion

Multiple ionization of the Ar$^{+}(3s^2 3p^5)$ ion by a single photon has been investigated in the photon-energy range 250-1800~eV employing the photon-ion merged-beams technique. Absolute partial cross sections were measured for all Ar$^{(1+m)+}$ product-ion channels with 1 $\leqslant$ m $\leqslant$ 6 covering a size range from several tens of Mb down to a few b. Narrow $2p$-subshell excitation resonances were observed in all channels up to quadruple ionization at a photon-energy bandwidth of 52~meV. Double excitations involving a $2p$ and a $3s$ or $3p$ electron were also studied at high resolution and the measurements of the broad $2s$ excitation resonances directly showed their natural widths. Contributions of direct photo double ionization (PDI) to the production of the highest final Ar ion charge states are revealed, with PDI of the $2s$ subshell being mainly responsible for the production of Ar$^{7+}$. The experiment made use of the PIPE setup installed at beamline P04 of the PETRA III synchrotron light source of DESY in Hamburg. The measurements were supported by theoretical calculations to identify the main contributions to the observed cross sections. Comparisons of theory and experiment show remarkable agreement but also hint to additional ionization mechanisms that are not considered in the theoretical models such as core ionization accompanied by excitations with subsequent Auger decays leading to net $m$-fold ionization with m $\leqslant$ 4