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High-resolution measurements of the Kα 1,2 X-ray spectra were performed with a bent crystal diffraction spectrometer for zirconium, niobium, molybdenum and palladium targets bombarded by 280 and 360 MeV oxygen ions. The energy differences between the Kα 1,2 diagram and the K h α 1,2 hypersatellite transitions as well as the intensity ratios I ( K h α 1 )/ I ( K h α 2 ) were determined. The L-shell satellites of the diagram and of the hypersatellite lines were resolved. The data are interpreted in terms of the properties of He-like hole states in mid- Z atoms. The Breit interaction correction is determined. The corrections due to the unresolved M-shell satellites and to the QED are found to be much smaller than the Breit terms and comparable to the experimental accuracy.

The radiative decay of double K-shell vacancies in zirconium (Z=40), molybdenum (Z=42), and palladium (Z=46) was observed using a high-resolution transmission crystal spectrometer. The samples were ionized by the impact of 100-MeV $^{4}\mathrm{He}^{2+}$ ions. From the well resolved ${\mathit{K}}^{\mathrm{\ensuremath{-}}2}$\ensuremath{\rightarrow}${\mathit{K}}^{\mathrm{\ensuremath{-}}1}$${\mathit{L}}^{\mathrm{\ensuremath{-}}1}$ transitions, the energies, linewidths, intensity ratios, and double K-shell ionization cross sections were deduced. The experimental energies and linewidths of the hypersatellites are compared with results of Dirac-Hartree-Slater calculations and with multiconfiguration-Dirac-Fock calculations, both taking into account the Breit interaction. For the extracted double K-shell ionization cross sections a comparison is made with the semiclassical approximation predictions within the independent-particle model.