Sara Lafuerza: The chemical sensitivity of core-to-core XES in 3d transition metals

K-fluorescence X-ray emission spectroscopy (XES) is receiving a growing interest in all branches of natural sciences to investigate the local spin in 3d transition metal complexes. Unlike the valence-to-core emission lines, the core-to-core transitions in Kβ (3p to 1s) and Kα (2p to 1s) do not probe the valence shell directly and the chemical sensitivity is thus indirect. In Kβ and Kα emission, the local spin sensitivity stems from the exchange interaction between the 3p (Kβ) or 2p (Kα) core-hole and the 3d valence shell spin of the transition metal ion, which is larger for Kβ than Kα [1]. The magnitude of the exchange interaction depends for a given element on the valence shell spin, which is defined by the metal atom oxidation and spin state within an ionic picture. This is a very crude description of the electronic structure and the influence of covalence in Kβ has been pointed out by several authors [1,2].
In this talk, the results of a systematic investigation of Kβ and Kα XES spectra measured on a wide range of iron compounds will be presented. More than 30 samples with different oxidation state (+2, +3, +4 and mixed-valence), spin (high-spin, low-spin and mixed-spin), ligands (fluorides, oxides, sulfides, etc.) or local coordination (octahedral, tetrahedral) were measured at beamline ID26 of the ESRF synchrotron. We analysed the experimental spectra in terms of commonly used quantitative parameters (Kβ1,3-first moment, Kα1-full width half maximum, and integrated absolute difference –IAD– [3]) and we carefully examined the difference spectra. We also performed multiplet calculations to elucidate the underlying mechanisms that lead to the chemical sensitivity.
Our results confirm a strong influence of covalency on both Kβ and Kα lines. We establish a reliable spin sensitivity of Kβ XES as it is dominated by the exchange interaction whose variations can be quantified by either Kβ1,3-first moment or Kβ-IAD and result in a systematic difference signal lineshape. We find an exception in the Kβ XES of Fe3+ and Fe2+ in aqueous solution, where a new difference spectrum is identified that cannot be explained by scaling the exchange integrals. We explain this with strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of Kβ XES spectral changes as due to spin variations without careful analysis of the lineshape. For Kα XES, the smaller exchange interaction together with the influence of other electron-electron interactions make it difficult to extract a quantity that directly relates to the spin.

References:

1.  P. Glatzel and U. Bergmann, Coord. Chem. Rev. 249, 65 (2005); https://doi.org/10.1016/j.ccr.2004.04.011
2. C. J. Pollock, M. U. Delgado-Jaime, M. Atanasov, F. Neese and S. De Beer, J. Am. Chem. Soc. 136, 9453 (2014); https://doi.org/10.1021/ja504182n
3.  G. Vankó, T. Neisius, G. Molnár, F. Renz, S. Kárpáti, A. Shukla, and F. M. F. de Groot, J. Phys. Chem. B 110, 11647 (2006); https://doi.org/10.1021/jp0615961