Atomic background in x-ray absorption spectra of fifth-period elements: Evidence for double-electron excitation edges

A systematic x-ray absorption study to detect atomic background features at the K edges of fifth-period elements is reported. Samples of MoCₗ (Z=42) and pure Rh (Z=45), Pd (Z=46), Ag (Z=47), In (Z=49), and Sn (Z=50) are measured at high temperature, mostly in the liquid phase, to reduce the intensity of the structural extended x-ray absorption fine structure (EXAFS) contribution. Main anomalies, showing a regular trend as a function of Z, are identified in the region around 100 eV and in the range 200--800 eV above an edge. They are assigned to the opening of absorption channels creating double-hole configurations such as 1s4p, 1s, 3d, and 1s, 3p. Differences of self-consistent calculations in the Dirac-Fock approximation have been performed and the dominant transitions unambiguously identified. The results are relevant for the improvement of the EXAFS analysis at these edges.