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This chapter presents TR-XAS measurement results in the liquid phase. This chapter presents TR-XAS measurement results in the liquid phase. By coupling a vacuum flat jet interaction chamber with the HHG water window source described in Chap. 2 and taking advantage of the cut-off extension presented in Chap. 4, the induced reaction is tracked simultaneously from the carbon and nitrogen K-edges at sub-100 fs time resolution. Measurements were performed on the liquid pyridine (C5H5N) excited by ultrashort NIR pulses in the high intensity regime and transient X-ray absorption near-edge structure (XANES) modifications were observed.

Time-resolved X-ray absorption spectroscopy has for long been experimentally limited to picosecond temporal resolution measurements in large-scale facilities. Recently, the expansion of high-harmonic generation table-top laser sources have led the femtosecond resolution of Ti:Sa oscillators to measurements in the extreme ultraviolet. This experimental chapter details the first results of the transient absorption setup developed in collaboration with the group of H.J. Worner of ETHZ that are described in Chap. 2. This experiment measures the dissociation of molecules in the gas phase from the sulfur L-edges and the carbon K-edge in the soft X-rays. The temporal resolution of the setup allows to follow light-induced chemical reactions of the lowest electronic states with a few femtoseconds resolution. By comparing the calculated core-level transitions with the observed splitting of orbital states, the assignment of resolved structures, in accordance with the literature, allows to reconstruct the geometry changes of the molecules along their reaction path.

This chapter presents the HHG driving pulse optimization results. The intensity and short pulse requirements to reach higher soft X-ray energies and better conversion ratio are discussed and different approaches are introduced. The previous, demanding, time-resolved X-ray absorption spectroscopy experiment in the gas phase leaned towards the option for an easy pulse shortening scheme. The adopted two-stage pulse broadening by filamentation coupled to a very simple post-compression technique is environmentally robust and leads to a 0.2 TW sub-two-cycle short-wavelength infrared pulse. The latter can drive high-order harmonics beyond the oxygen K absorption edge with similar flux conditions as obtained during the successful gas dissociation measurements at the carbon K-edge.

This chapter introduces in detail the different elements assembled and designed to get the soft X-ray transient absorption experiment ready. Some maintenance routines and operational specifications naturally stem from the laser source’s reliance on a commercial system. It describes and details every pulse involved in the experiment, as well as the optical post-treatments applied to the pump pulse and the high harmonic generation driver to increase the system overall performances. The condition of soft X-ray generation in helium with long wavelength lasers is discussed and compared to the most advanced similar systems, providing an overview of the state of the art for tabletop time-resolved absorption spectroscopy in the water window. The characteristics of the flat-field vacuum spectrometers that were used to measure the signals over the full water window are also presented and discussed in detail. Finally, the data acquisition and data analysis methods are provided.