Photoionization is at the heart of X-ray photoelectron spectroscopy (XPS), which gives access to important information on a sample's local chemical environment. Local and non-local electronic decay after photoionization-in which the refilling of core holes results in electron emission from either the initially ionized species or a neighbour, respectively-have been well studied. However, electron-transfer-mediated decay (ETMD), which involves the refilling of a core hole by an electron from a neighbouring species, has not yet been observed in condensed phase. Here we report the experimental observation of ETMD in an aqueous LiCl solution by detecting characteristic secondary low-energy electrons using liquid-microjet soft XPS. Experimental results are interpreted using molecular dynamics and high-level ab initio calculations. We show that both solvent molecules and counterions participate in the ETMD processes, and different ion associations have distinctive spectral fingerprints. Furthermore, ETMD spectra are sensitive to coordination numbers, ionsolvent distances and solvent arrangement.Site-selectivity and sensitivity to the local chemical environment have made X-ray photoelectron spectroscopy (XPS) a powerful tool for probing both gas phase and condensed matter-its development has also led to a deeper understanding of the complex and competitive processes that occur as a result of X-ray-substrate interactions. The creation of deep inner-shell electron holes through X-ray photoionization is followed by relaxation processes that provide additional important insight into electronic structure and correlation in the valence-electron region. One such process is Augerelectron decay, in which, within the same initially ionized species, a valence electron relaxes to fill a core vacancy, causing the emission of an electron from a higher state. Auger processes have found widespread applications in many areas of research, especially in materials science and surface-composition analysis. Element-selectivity of these X-rayinduced de-excitations opens a way for targeted energy deposition, which can be used in medicine, in particular for cancer treatment1 or for selective transformations of molecules and materials2. However, Auger decay is not the only relaxation process that can occur after initial photoelectron emission. Several experimental and theoretical works have demonstrated electronic relaxation processes that can efficiently compete with local Auger decay and that are 'non-local' in nature, that is, they involve species other than the initially ionized monomer. The best studied process is intermolecular Coulombic decay (ICD)3, which occurs in weakly interacting systems such as rare gases and hydrogen-bonded complexes4,5. In an ICD process, the energy gained after refilling the initial hole created by ionization or excitation is used to eject an electron from a neighbouring species, resulting in the formation of two singly charged units that subsequently separate by Coulomb repulsion. The competition of non-local an...
