X-ray spectroscopy of complex chemical systems in liquid phase

Modern light sources like synchrotron and free electron laser facilities provide high brilliant, monochromatic and high coherent X–ray beam with tunable photon energies covering almost the whole electromagnetic spectrum. These unique properties of the light sources enable to implement novel experimental methods for investigating dynamical chemical processes. The atomic components of chemically relevant systems and solvents essentially consist of light elements. The chemical reactivity of these systems are primarily defined by their electronic configurations. Information about the electronic structure would elucidate the understanding of chemical processes and reactions on a fundamental level. This is one prerequisite to obtain a complete picture of the chemical characteristics. Aqueous solutions and isomerization processes are elemental phenomena in nature. However, the description of these mechanisms on the molecular level is not complete. In this thesis, static and time–resolved X–ray spectroscopic methods are described which have been developed and applied to investigate fundamental chemical interactions of aqueous solutions and isomerization of molecules on the atomic scale. The work of my PhD thesis consists of designing and commissioning an experimental end station including a detection device to perform the aimed experiments in liquid phase. The goal is to investigate ion– water interactions as well as isomerization processes. By employing the X–ray spectroscopic technique, the unoccupied and occupied orbitals are probed and information about the electronic configurations are obtained. Concerning the ion–water interaction, the results showed that the water molecules are mainly influenced by the cation and that ion specific influences are mirrored in the electronic structure. Furthermore, the impact of ions is primarily limited to water molecules in close vicinity. A significant difference between the first solvation shell and bulk water is observed. The results of the second phenomenon investigated have shown remarkable differences in the electronic configuration of trans– and cis–azobenzene. Furthermore, with X–ray spectroscopy all electron– hole–pair final states are observed, including the optically dark states. This fundamental work might help to further elucidate the dynamical pathways of the isomerization mechanism. Additionally, the first time–resolved soft resonant inelastic X–ray scattering (RIXS) experiment at a synchrotron was carried out successfully in an optical pump and X–ray probe scheme. The future direction and the opportunities of this relatively young research field of complex chemical systems in liquid phase will be addressed.