Applications of resonant and non-resonant core-level photoelectron (PE) spectroscopy to liquid microjets for the study of the electronic structure of liquid water and aqueous solutions are presented. Exemplified for transition metal ions in water, non-radiative relaxation processes following resonant 2p excitation are shown to uniquely reveal details on the bonding interactions of the metal-aqua complexes. Here the interaction between specific water orbitals and metal-derived 3d orbital can be identified from electron signal enhancement at the characteristic valence electron binding energies. This information is the prerequisite for the interpretation of electron-yield X-ray absorption spectra.
Another unique aspect of core-level photoemission / relaxation spectroscopy from aqueous phase is the shape of Auger-electron spectra. The important spectral feature is the high-kinetic energy tail of the Auger spectrum, which has no gas-phase analogue, and hence reflects the participation of solvent water in the relaxation process. For instance, O1s Auger electron spectroscopy from liquid water reveals a novel electronic de-excitation process of core-level ionized water in which a pair of two cations forms, either H₂O⁺···H₂O⁺ or OH⁺···H₃O⁺. These reactive species are the delocalized analogue to H₂O²⁺, formed in a localized on-site Auger decay, and are expected to play a considerable role in water radiation chemistry and biodamage. Both cationic pairs form upon autoionization of the initial ionized water molecule. Isotope measurements show that autoionization also occurs from a series of transient Zundel-type structures evolving from proton transfer, from the ionized water molecule to a neighbor molecule, within a few femtoseconds. The actual autoionization is either through intermolecular Coulombic decay (ICD) or Auger decay. These so-called proton-transfer mediated charge separation (PTM-CS) processes are found to also occur in other and similarly hydrogen-bonded solute molecules such as NH₃ (aq), NH₄⁺ (aq), or H₂O₂ (aq). Their probabilities strongly correlate with the hydrogen-bond strength which makes autoionization spectroscopy suitable for the characterization of hydration structure