![]() ![]() Since bending, on the one hand, improves the photon efficiency but, on the other, reduces the energy resolution, a relatively large bending radius has to be used. They typically employ a curved crystal geometry (Johann, 1931 von Hámos, 1932 ). For this purpose, wavelength-dispersive spectrometers are used. Typically, CtC transitions are used for both resonant inelastic X-ray scattering (RIXS) and HERFD-XAS due to the high transition probability, which is beneficial for recording weak RIXS signals.įor simultaneous X-ray emission-line measurement with high energy resolution, the dispersion of X-rays by a crystal or crystal array is mandatory. Moreover, XES is the technical basis for resonant X-ray emission spectroscopy studies (RXES) (Glatzel et al., 2009 Szlachetko et al., 2013 ) and high-energy resolution fluorescence detected XAS (HERFD-XAS) (Atkins et al., 2012, 2015 ), the latter demanding an experimental resolution that is lower than the natural lifetime broadening of the excited state. ![]() As such, both methods are capable of removing certain limitations of classical X-ray absorption spectroscopy concerning the discrimination of coordinating atoms that are close neighbors in the periodic table and the determination of oxidation states. In contrast, in CtC-XES, the relaxation of a core electron from the 2 p or 3 p level is recorded, which yields a detailed view on the localized d-electron density due to the exchange interaction with the d states (Glatzel & Bergmann, 2005 ). Therefore, VtC-XES is highly sensitive to the ligands coordinating to a metal center in complexes or the coordinating ions in solid-state materials. In VtC-XES, the 1 s electron is non-resonantly excited into the continuum, and the radiative HOMO (highest-occupied molecular orbitals) to 1 s channel is detected. Both methods have found many applications in chemistry (Zimmer et al., 2017 Delgado-Jaime et al., 2013 ), geology (Weis et al., 2019 Atkins et al., 2015, 2012 ), catalysis (Singh et al., 2010 ), magnetic materials (Vankó et al., 2007 ) and many more (Bauer, 2014 ). High-energy-resolution non-resonant hard X-ray emission spectroscopy (XES) is increasingly recognized as a powerful tool to determine the electronic and even geometric structure of matter by means of core-to-core (CtC) and valence-to-core (VtC) XES. ![]()
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