Intermolecular electronic decay in aqueous solutions

This thesis reports on ground state and excited state electronic-structure studies from water and aqueous salt solutions by means of liquid-jet soft-X-ray photoemission (PE) spectroscopy. Novel PE spectroscopy methods and detection schemes are utilized in order to investigate changes in electronic structure, ion-solvation and nuclear dynamics upon soft-X-ray irradiation. The first part of the thesis addresses the influence of atomic ions on the electronic structure of water. Hydrated ions have a profound effect on the geometric structure of liquid water and changes in the electronic structure are intuitively expected. However, using direct photoelectron spectroscopy, it will be shown that even very high salt concentrations have a negligible effect on the liquid water electronic structure. An almost unchanged PE spectrum is observed when going from neat water to 8 molar NaI concentration, which is argued here to be a result of the highly efficient screening of the ionic charges by the polarizable water molecules. Besides the direct photoelectron experiments, novel spectroscopy methods are presented, exploiting non-local autoionization processes upon core-ionization. These electronic decay pathways involve energy- and electron transfer between neighboring molecules. Two types of nonlocal decay mechanisms are presented, intermolecular Coulombic decay (ICD) and electrontransfer mediated decay (ETMD), which are shown to be general phenomena in weakly interacting systems, such as hydrogen-bonded networks. The characteristics of these two decay channels can be used to infer information about solvation structure and dynamical processes in aqueous solution upon core-level ionization. In particular, I report on proton nuclear dynamics triggered by soft-X-ray irradiation of the aqueous ammonium cation, NH+4 (aq). The probability for ICD between N 1s ionized ammonium and surrounding water molecules scales with hydrogen-bond strength. PE spectral changes when comparing normal ammonium and its deuterated form, ND+4 (aq), are related to altering nuclear dynamics along the N–H/D coordinate. Most notably, it is observed that a complete proton transfer between ammonium and a coordinated water molecule proceeds within the ultrashort lifetime of the N 1s vacancy (~7 fs). This complete chemical reaction is even accompanied by a second, although incomplete proton transfer to a second water molecule. Very highly concentrated aqueous solutions have usually a high abundance of associated ion pairs, which possibly enables non-local autoionization pathways solely involving ions while neglecting the solvent water. I show that ETMD spectroscopy is very sensitive to local molecular arrangement due to its extreme short-range dependence. It is possible to directly extract ion-pairing character – be it separated, solvent-shared or contact pairing – from ETMD spectral analysis. However, experimental detection of low-energy ETMD electrons is very challenging when using standard PE spectroscopy. For the first time, the electron–electron coincidence detection scheme was applied to a liquid jet. This allowed to record Li 1s ETMD spectra from lithium aqueous solution, Li+ (aq), with enhanced collection efficiency compared to conventional detection schemes. ETMD spectroscopy’s unique ion-pair sensitivity is proven by changes in the PE spectral shapes corresponding to ETMD when comparing LiCl versus LiCOOCH3.