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...