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Fe X-ray emission spectra (XES) of diferric (red), mixed-valent (blue), and diferrous (black) Fe2S2 clusters: mainline (top left) and second-derivative spectra (bottom left; smoothed with second-order Savitzky–Golay algorithm); valence-to-core (top right) and second-derivative spectra (bottom right). Derivative spectra were smoothed using the second-order polynomial Savitzky–Golay algorithm in order to obtain better peak definition.
