Probing Coster–Kronig Transitions in Aqueous Fe²⁺ Solution Using Inverse Partial and Partial Fluorescence Yield at the L-Edge

Specific Coster–Kronig (CK) transitions in 3d transition metals are close to the threshold of energetic possibility, being disallowed in free atoms, while possible in solids. Moreover, they have been shown to be quite sensitive to chemical bonding. Nevertheless, there has been no direct study of such behavior in solution. Here we present an approach to quantify such transitions in solution, by comparing relative fluorescence of different edges to their relative absorption strengths. The difficulties of acquiring a measurement of the absorption in solution are overcome by applying the recently developed method of inverse partial fluorescence yield to a liquid sample using the microjet. This method has been demonstrated on solids to be bulk sensitive and able to obtain absorption spectra free of self-absorption or saturation effects. We extend this approach to investigate the L-edge of aqueous Fe²⁺ using a combination of a soft X-ray light source and a high-resolution X-ray emission spectrometer