Enabling the Study of Aqueous Solvation Dynamics with Ultrafast X-ray Scattering

X-ray radiation from X-ray Free electron Lasers (XFELs) provides access to atomic dimensions, with spatial resolution reaching below 10−10 m. Moreover, the ultrashort nature of XFEL pulses allows capturing dynamic processes occurring on timescales as short as 10−14 s. This unique combination of spatial and temporal resolution empowers exploration of molecular interactions. This thesis investigates the application of Ultrafast Time-Resolved X-ray Solution Scattering (TR-XSS) for studying solvation dynamics. While these phenomena have been studied extensively at XFELs in the United States, Japan, and South Korea, the newer XFEL facilities in Europe are still in the process of demonstrating their TR-XSS capabilities. This work investigates the TR-XSS potential of both SwissFEL and the European XFEL.
The first part of this thesis contains an analysis of TR-XSS data, from SwissFEL, measured on an iron complex in aqueous solution. [Fe(bpy)(CN)₄] ⁻² , is a model system for materials with the potential to be utilized in solar cell technologies. The efficiency of this complex, quantified by the lifetime of its first excited state, exhibits significant variations depending on the solvent environment. Comparing the measured data to both steady state and non-adiabatic  simulations, the structural changes occurring during the complex’s photocycle are illuminated in attempt to understand the solvent dependent mechanism.
Secondly, the TR-XSS capabilities of the European XFEL are explored. At all XFELs the intense X-ray pulses introduce sample perturbations. As a result of the unique pulse structure at the European XFEL, these perturbations, lead to systematic artefacts in the difference scattering data. The analysis presented here investigates the precise origin of these artefacts and highlights strategies to mitigate their impact, ensuring reliable TR-XSS measurements.
Lastly, non-standard detector gain configurations at the Linac Coherent Lightsource (LCLS) are investigated. The higher energies and improved time resolutions that are becoming available at XFEL facilities come at the cost of X-ray flux. This fact, combined with the push to study low-Z organic molecular systems makes accurate photon detection even more important than previously. By tailoring gain settings, the signal-to-noise ratio is enhanced, while avoiding saturation in the high intensity regions of the detector. I explore the possible downsides of such configurations in the form of non-linearities in the gain response.
This thesis work underscores the importance of rigorously characterizing X-ray detectors and other experimental equipment under actual experimental conditions as well as the requirement for beamline and experiment specific corrections and filtering.