Electronic structure of biomolecules in aqueous solutions: Photo/Auger-electron spectroscopy from a water microjet using synchrotron light

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Our follow-up liquid-jet photoelectron (PE) spectroscopy studies of the electronic-structure interactions in aqueous solutions will increasingly explore charge and energy transfers between biologically relevant molecules and the surrounding solvent water molecules. Soft X-rays (<1600 eV) from the BESSY synchrotron-radiation facility are used for ionization and/or excitation. Core-level excitation and the subsequent ultrafast refill of the core hole leads to emission of Auger-type electrons. Often, electronic relaxation will not be local but rather involves charge or energy transfer between solute and water solvent. This can be identified through secondary (or autoionization) processes, and will be investigated here. In recent studies we have pioneered these routes for neat liquid water and for several aqueous solutions, including OH-(aq) and transition metals(aq). The strength of a given solute – water(shell) interaction defines the extent of orbital overlap and also defines the solvation configuration, and together this determines the details of the electronic relaxation pathway following local excitation. Our studies specifically focus on spectator Auger decay, intermolecular Coulombic decay (ICD), and resonant photoemission (RPE) spectroscopy, now for the first time applied to DNA components and other biologically relevant molecules, including metal-containing molecular complexes in water. Complementary non-resonant PE studies provide core-level and valence electron energies (specifically lowest ionization), the latter being crucial for a better understanding of aqueous solution chemical reactivity. In addition, core-level energy shifts reveal accurate atomic-site energy changes associated with chemical changes of the environment, induced for instance by change of pH value.