Chemical Sensitivity of Kβ and Kα X-ray Emission from a Systematic Investigation of Iron Compounds

K-fluorescence X-ray emission spectroscopy (XES) is receiving growing interest in all fields of natural sciences to investigate the local spin. The spin sensitivity in K beta (K alpha) XES stems from the exchange interaction between the unpaired 3p (2p) and the 3d electrons, which is greater for K beta than for K alpha. We present a thorough investigation of a large number of iron-bearing compounds. The experimental spectra were analyzed in terms of commonly used quantitative parameters (K beta(1,3) -first moment, K alpha, full width at half-maximum, and integrated absolute difference -IAD-), and we carefully examined the difference spectra. Multiplet calculations were also performed to elucidate the underlying mechanisms that lead to the chemical sensitivity. Our results confirm a strong influence of covalency on both K beta and K alpha lines. We establish a reliable spin sensitivity of K beta XES as it is dominated by the exchange interaction, whose variations can be quantified by either K beta(1,3)-first moment or K beta-IAD and result in a systematic difference signal line shape. We find an exception in the K beta XES of Fe3+ and Fe2+ in water solution, where a new difference spectrum is identified that cannot be reproduced by scaling the exchange integrals. We explain this by strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of K beta XES spectral changes as due to spin variations without a careful analysis of the line shape. For K alpha XES, the smaller exchange interaction and the influence of other electron electron interactions make it difficult to extract a quantity that directly relates to the spin.