Ion pairing in electrolyte solutions remains a topic of discussion despite a long history of research. Very recently, nearest-neighbor mediated electronic de-excitation processes of core hole vacancies (electron transfer mediated decay, ETMD) were proposed to carry a spectral fingerprint of local solvation structure and in particular of contact ion pairs. Here, for the first time, we apply electron-electron coincidence detection to a liquid microjet, and record ETMD spectra of Li 1s vacancies in aqueous solutions of lithium chloride (LiCl) in direct comparison to lithium acetate (LiOAc). A change in the ETMD spectrum dependent on the electrolyte anion identity is observed for 4.5 M salt concentration. We discuss these findings within the framework of the formation and presence of contact ion pairs and the unique sensitivity of ETMD spectroscopy to ion pairing.

Department of Physical Chemistry University of Chemistry and Technology Technická 5 16628 Prague Czech Republic

Department of Physics Freie Universität Berlin Arnimallee 14 14195 Berlin Germany

Helmholtz Zentrum Berlin für Materialien und Energie Methods for Material Development Albert Einstein Str 15 12489 Berlin Germany

Leibniz Institute of Surface Modification Permoserstr 15 04318 Leipzig Germany

Max Planck Institute for Plasma Physics Wendelsteinstr 1 17491 Greifswald Germany

School of Chemistry Monash University 3800 Clayton Victoria Australia

Wilhelm Ostwald Institute for Physical and Theoretical Chemistry University of Leipzig Linnéstr 2 04103 Leipzig Germany

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