Stability, Structure and Photochemistry of Water Clusters

Water is the most important solvent on Earth. With its unique properties, it enables biological processes. The water circle, including cloud condensation, plays a key role for the climate, and changes in nucleation rate have pronounced impact on cloud formation. Technologically, water is an environmentally benign solvent for chemical processes, and the raw material for carbon-neutral hydrogen production. In our research project, we address fundamental physical and chemical properties of water with clusters of up to about 100 water molecules. Such small clusters can be studied in a mass spectrometer, and chemical change is tracked by measuring the mass of the cluster at a given time during the experiment. The charge carrier, in our case a proton or an excess electron, changes the structure of the water network in its vicinity. We investigate the stability of the water network for these two charge carriers, with a focus on cluster sizes that exhibit particularly high intensity in the mass spectrum. These magic cluster sizes are believed to represent structures of special stability. We tune the temperature of the environment, measure the lifetime of the cluster against evaporation of water molecules, and develop computer models for this reaction. To this end, we use advanced quantum chemical methods to solve the Schrödinger equation for all electrons in the cluster, and to describe the oscillations of the nuclei. In this interplay of experiment and theory, we develop advanced statistical models for the evolution of water clusters in a thermal radiation environment. In a second series of experiments, we include a partially oxidized metal ion in the water cluster, which leads to the evolution of hydrogen. This enables us to expand the computational models to hydrogen evolution reactions, which are of key importance for the large-scale production of hydrogen from clean energy sources. We will also use advanced laser systems to prepare the water clusters in different quantum states and trigger chemical reactions, in particular hydrogen evolution reactions, with light. For these experiments, a precise quantum chemical description of the quantum states involved is essential to understand the chemical reactions in electronically excited states.