First Observation of Two-Electron One-Photon Transitions in Single-PhotonK-Shell Double Ionization

Abstract

Experimental evidence for the correlated two-electron one-photon transitions (1s À2 ! 2s À1 2p À1 ) following single-photon K-shell double ionization is reported. The double K-shell vacancy states in solid Mg, Al, and Si were produced by means of monochromatized synchrotron radiation, and the two-electron one-photon radiative transitions were observed by using a wavelength dispersive spectrometer. The two-electron one-photon transition energies and the branching ratios of the radiative one-electron to twoelectron transitions were determined and compared to available perturbation theory predictions and configuration interaction calculations.Understanding electron-electron interactions in ionization, excitation, and relaxation of many-body systems is one of the key issues of atomic physics. In this context of special interest are hollow atoms, i.e., atoms with empty innermost shells and occupied outer shells, because singlephoton double K-shell ionization is driven by multielectron interactions (see [1,2], and references therein), and the decay of K-shell hollow atoms involves electron correlation effects. Furthermore, hollow atom formation in ultraintense hard x-ray free-electron laser beams reveals electron dynamics on the femtosecond time scale [3].In response to a K-shell doubly excited state, electron relaxation and rearrangement processes follow. The excited atom decays in a cascade of nonradiative Auger and radiative transitions. The radiative decay of double K-shell hole states proceeds mainly through the oneelectron one-photon (OEOP) process, which corresponds to the K h ð1s À2 ! 1s À1 2p À1 Þ hypersatellite transition. In the few orders of magnitude weaker competitive decay channel, the two-electron one-photon (TEOP) transition K h ð1s À2 ! 2s À1 2p À1 Þ, the two K-shell core holes are filled simultaneously via a correlated two-electron jump and one photon is emitted (see Fig. 1). TEOP transitions are thus correlated multielectron processes which can be described only by many-electron models.Interest in TEOP transitions dates back to 1925. Predicted by Heisenberg [4], it was only 50 years later that the first experimental evidence for TEOP transitions in heavy-ion (HI) collisions was reported by Wölfli et al. [5]. Although the K h to K h branching ratio should not depend on the excitation mode, multiple electron ionization in HI collisions changes the electronic configurations and affects the intensities and energies of the measured transitions. Thus data from HI collision experiments show a wide spread of values [6][7][8][9][10], making comparison with theory often inconclusive. On the theoretical side, significant differences in the predicted TEOP radiative decay rates have been reported [11][12][13][14][15][16][17][18][19][20][21]. In this respect, photon impact data provide a more stringent test for atomic structure calculations. However, the single-photon double K-shell ionization cross sections are $10 3 smaller than in HI collisions. Thus photoionization experiments are more challenging, and, to t...

Photoinduced K-shell hollow atoms

“…The energies and linewidths of the K˛h hypersatellite X-ray transitions and the K˛h 1 /K˛h 2 intensity ratios are reported and compared to theoretical calculations as well as to other available experimental data. For Mg, Al and Si, the correlated two-electron one-photon transitions in single-photon K-shell double ionization could be observed recently for the first time [23]. The TEOP energies and K˛h to K˛˛h branching ratios obtained in this experiment are also presented and discussed.…”

“…In counterpart, single-photon double K-shell ionization cross sections are 10 2 -10 3 smaller than in HI collisions. Thus photoionization experiments are more challenging and, to the best of our knowledge, all attempts to measure photoinduced TEOP transitions have been unsuccessful (see e.g., [22]) until very recently [23]. Note that the TEOP analogous KK-LLX (X = L, M, etc.)…”

Confocal X-ray technology based on capillary X-ray optics

“…According to the way of obtaining characteristic XRF spectrum, the XRF analysis method can be divided into a wavelength dispersive mode and an energy dispersive mode. Confocal XRF is most the energy dispersive mode (Woll et al 2006, Samber et al 2008, Sun et al 2011) and can also be designed into the wavelength dispersive mode (Hoszowska et al 2011). For all the available confocal XRF facilities based on CXRO, the focusing X-ray optics in the detection channel is PXRO or MXRO and the focusing X-ray optics in the excitation channel may be MXRO, PXRO, compound refractive lenses (CRLs), Kirkpatrick-Baez (KB) mirrors, Fresnel zone plates, and so on.…”

Deep core ionic photofragmentation of the CF2Cl2 molecule

“…Thus, the study of molecular fragmentation after the absorption of a single photon allows one to obtain knowledge on the coupling among the initial and final target states. For instance, experimental evidence for the correlated two-electron one-photon transitions following single-photon K-shell double ionization was recently reported for the first time [1].…”