Ultrafast Dynamics: Time-resolved Measurements
Our research focuses on the study of ultrafast phenomena in solids and at surfaces by employing time resolved soft X-ray core level spectroscopies (X-ray Emission Spectroscopy, XES, and Photoelectron Spectroscopy, PES) to measure the time resolved and atom specific evolution of the electronic structure of the system.
In particular we employ pump-probe techniques: a pump pulse drives the system under study out of equilibrium and a delayed X-ray probe pulse takes snapshots of the system evolution at different times (Δt).
As a probe, we use short pulse high intensity soft X-rays delivered by FELs (FLASH and LCLS) or, in the future, table top lasers. As pump, we employ optical laser pulses (400/800 nm) synchronized with the FEL or X-rays obtained by splitting the FEL radiation with a beam splitter.
Chemistry at surfaces
Most of the industrially relevant chemical reactions occur at surfaces and interfaces and one of the great challenges in surface chemistry is to capture and ultimately control the transition states active in surface reactions. By using time resolved XES and PES we monitor the electronic and geometric structural changes along the reaction path, probing therefore the reaction dynamics on the femtosecond time scale. This enables us to obtain a full picture of surface mediated chemical reactions.
We have studied by means of time resolved XES the liquid-liquid phase transition in silicon  and we are currently studying (within a „Surface Science collaboration” including several international institutes) the desorption and oxidation of CO molecules from Ru(0001) triggered by an optical laser pulse (400nm).
The delicate interplay between spin, charge and lattice degrees of freedom in correlated materials yields rich phase diagrams as function for example of chemical composition and external fields. The proximity of various phases, especially near a quantum critical point, can originate electronically distinct domains into the material which experience temporal and spatial fluctuations over many time and length scales. We employ time resolved core level spectroscopies to study the evolution of such systems.
We have studied the TaS2 charge-density-wave metal insulator transition induced by optical excitation  and we are currently studying the ultrafast demagnetization of iron films induced by ultrashort optical laser pulses (800nm) (in collaboration with Yves Acremann, ETH Zurich). The aim is to obtain a microscopical picture for such ultrafast changes.