Spectroscopy and dynamics at liquid water interfaces

This thesis is a detailed description of three experimental investigations on aqueous interfaces. All projects made use of the microjet technology or the more recently developed flat-jet technique which enables the implementation of liquid water in vacuum chambers. In the first study presented here we show that a flat-jet created from the impingement of two liquid microjets generates a laminar turbulence-free water-water interface. By colliding a Luminol solution microjet with another microjet of hydrogen peroxide solution, the chemiluminescence of Luminol reveals where the liquids mix in the flat-jet. The liquids readily mix in the rims of the flat-jet, while in the middle part the different liquids only meet via diffusion. Flat-jets are stationary systems such that the longitudinal flow direction translates into a time dimension, providing access to the interface in different total interaction times ranging from 10 to 200 µs. This timescale is difficult to access by current techniques. The water-vacuum interface is the next topic, in which we have studied aqueous solutions using microjets and photoelectron spectroscopy (MJ-PES). For the first time, we have experimentally demonstrated the implementation of an absolute photoelectron energy reference in MJ-PES. We could show that there is a concentration-dependent shift of valence bands which were previously (without this absolute energy reference) undetectable. Even though this study was done using a tabletop He plasma light source, it creates the foundation of absolute energy referencing for MJ-PES works done in, e.g., synchrotron laboratories. Lastly, we have built an experimental apparatus to investigate the water-gas interface. Such interfaces are ubiquitous in nature but lack direct experimental works on the molecular scale, namely scattering studies. This is largely due to the technical difficulty imposed by the high vapour pressure of water. The dense layer of vapour shields the liquid surface in such a way that conventional molecular beams cannot travel to and from the liquid surface without interacting with the vapour phase, destroying the initial and final states required to understand the surface scattering event at the quantum level (transmission probability of ~e^(-100)). We propose an alternative method to this: by approaching the molecular beam source and the water flat-jet to a distance of 200 µm or less, the probability of scattering solely at the liquid surface becomes realistic. We have performed preliminary experiments using the developed apparatus.