ESCA (Electron Spectroscopy for Chemical Analysis, also known as X-ray photoelectron spectroscopy) has become a powerful conventional analytical method for solids and gases. We are expanding the technique to the realm of liquids. We focus on establishing the method as an analytical tool in chemistry, highlighting its surface-sensitivity, ultrafast nature, and ability to provide a full electronic structure of liquid-phase molecules.

The world of liquids exhibits various X-ray-initiated electronic relaxation processes that lead to electron emission and can be, therefore, also recorded in our measurements. Some of these processes are unique to liquids. Moreover, they are extremely specific, as they involve the nearest neighboring molecules. This provides us with an unprecedented analytical probe of coordination and a weakly bonded environment. We use these processes to study biological interactions and the solvation of molecules.

Using photoemission spectroscopy, we study ion pairing in solutions. This is important in various fields, ranging from biological sensing via receptors to processes in ion batteries. We systematically explore the effect of the size, charge, geometry, and concentration on forming ion pairs. The topic is closely connected to studying solvation shells around molecules or ions. We focus on disentangling specific solvation patterns and their effect on the measured binding energies.

We study biologically interesting acid–base systems in pH-dependent measurements. Specifically, we utilize the ultrafast nature of the photoionization process to probe the proton sharing and localization vs. delocalization. This is important in enzyme-driven metabolic pathways such as the citric acid cycle.