Using monochromatized extreme-UV radiation from the undulator beamline BW3 at HASYLAB we have measured at high resolution (E/ Delta E approximately 7000) the Li+ photoion yield spectrum at excitation energies corresponding to transitions to double K-shell vacancy states. A number of such resonances including single-photon-three-electron excitations, e.g. 1s22s(2S) to 2p3(2p), have been observed for the first time. The spectrum is tentatively classified with the aid of Hartree-Fock calculations.

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An atomic beam of tungsten and photoexcitation with monochromatized synchrotron radiation were used to measure the first photoion yield spectra of W+ and W + in the region of 30 -60 eV. The comparison with calculated photoionization cross sections shows that the resonance structure can be explained by discrete Sp~Sd, 6s and 4f~Sd transitions if one takes into account the thermal population of the initial states 5d 6s Dp & 234 and 5d 6s 53.

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Using synchrotron radiation, we have determined the absolute cross section for double photoionization involving the 2p and 3s electrons in atomic sodium. %e have also calculated this cross section in the many-body perturbation theory approximation.Our experimental and theoretical results are in good agreement one with each other and represents the first absolute determination of a double photoionization cross section in the case where the two electrons belong to two different shells.PACS numbers: 32.80. Hd, 32.80.Fb In this Letter, we present the results of the first experimental determination of an absolute double photoionization cross section for the direct emission into the continuum of two electrons belonging to two different shells. These measurements were performed for simultaneous photoionization of 2p and 3s electrons in sodium atoms. We also present calculations of the double photoionization cross section of Na (2p3s kik2) for photon energy from 50 to 250 eV, using many-body perturbation theory (MBPT).Double photoionization (DPI) in the outer shell of atoms has long attracted considerable interest, because it provides a sensitive test for the importance of electronelectron correlation effects. Earlier work on the rare gases [1 -4] revealed that the shakeoff theory, describing DPI as the result of monopole transitions resulting from relaxation of the hole state produced in the outer shell, was unable to account for most of the DPI intensity. MBPT was used at that time to calculate DPI cross sections in neon [5 -7], helium [8], and argon [7]. Agreement with the first experimental data available was reasonably good, especially for helium [9]. Most recently, DPI has been extensively studied in the outer shell of the rare gases [10 -18], owing to the increased performance of synchrotron radiation (SR) sources. In the measurements of the energy dependence of the branching ratio between double and single photoionization cross sections, considerable attention was given to the threshold [10-12] and high energy [13 -15] behaviors of DPI. Some data have also been obtained in the study [16 -18] of the energy and angle-resolved DPI process. Parallel to this experimental activity, progress has also been made in theoretical calculations of DPI cross section for helium [19 -22] and for argon [23].When the primary vacancy is produced in an inner atomic subshell, doubly charged ions can result from two different mechanisms.In the one-step DPI, a second electron is simultaneously ejected from the outer shell via correlation effects, namely [5 -8] core relaxation (CR), ground state correlations (GSC), and inelastic scattering (IS) of the photoelectron in collision with outer shell electrons while leaving the atom. CR and IS can be grouped into the category of final state correlations (FSC), although such a classification should be considered with some caution, as it has been shown that it is gauge dependent [19]. In the two-step process [24], the ejection of the first electron is followed by Auger decay of the singly c...