## 3 Chemistry Applications under Ambient Conditions
### Towards Femtosecond XANES and Advanced Spectroscopic Techniques  
Femtosecond XAS at FXE (similar to XES) is designed to be performed in two principal modes: scanning and single-shot. For the scanning configuration, FXE is equipped with a 4-bounce fixed-exit Si(111) monochromator based on 2 pairs of cryogenically cooled quasi channel-cut crystals. The detection of absorption spectra will be performed with fast PIN diode or APD (avalanche photodiode)  
Figure 6: Pump-probe difference WAXS signal from an aqueous solution of [Fe(bpy)\({}_{3}\)]\({}^{2+}\) collected at the ESRF synchrotron with 25.2 keV X-ray photons at 100 ps time delay. The experimental curve (cyan circles and green trace in the top inset) was fitted using a LCF and solute structure delivered by the AIMD (red curve) or DFT (\({}^{5}\)S-D\({}_{3}\) state, black curve). The high spatial resolution enabled by the large \(q\)-range allows for selecting the best suited structural models of the solution (see top inset) and better decorrelation of the solute (excited state fraction) and the solvent (bulk temperature rise) observables as compared to the case of the limited \(q\)-range (see the bottom inset). point detectors, either in total fluorescence yield (TFY) mode, high energy-resolved fluorescence detection (HERFD) or transmission mode, depending on the geometry, sample concentration and chemical element (absorption cross section) of interest. By adjusting the FEL undulator gap synchronously with the monochromator energy setting the baseline capabilities of performing standard 20-50 eV XANES, spectroscopy at FXE is enhanced to the possibility of recording EXAFS over hundreds of eV. The femtosecond time resolution can be preserved by adapting the laser time delay to the varying X-ray pulse arrival times during such monochromator scans, which changes the X-ray beam path length through the monochromator in a predictable way, i.e., continuously increasing the photon energy geometrically leads to shortening of the beam path through the 4 bounces due to the decreasing Bragg angle, thus shifting the X-ray pulse arrival to earlier times. Both X-ray absorption techniques are indispensable for probing the electronic and local structural dynamics during the course of ultrafast photochemical processes [45; 46; 47; 48; 54]. The single-shot X-ray absorption configuration is based on two identical curved-crystal spectrum analyzers which are positioned upstream and downstream of the sample interaction region and equipped with one-dimensional Gotthard detectors to simultaneously measure the dispersed SASE spectra on a single-shot basis even at pulse arrival rates in the MHz range. Both spectrum analyzers have identical sets of two Si ((111) and (110) cuts), one Ge (110) and one 20 \(\upmu\)m thick diamond (111) bent crystals to cover different energy ranges with selected resolution and efficiency over the 5-20 keV range. Only the diamond crystals can be used non-invasively in the direct beam while the Si and Ge crystals are operated with the first order beam behind a transmissive diamond diffraction grating installed upstream in the tunnel part of the beamline. Such correlated single-shot measurements using both spectrum analyzers allow us to quickly accumulate X-ray absorption spectra with the \"pink\" SASE beam (up to 0.2% relative bandwidth) near the selected absorption edge [55; 56] within the entire X-ray photon energy range covered by the instrument (5-20 keV). In addition to the information content due to the element-specific chemical sensitivity provided by X-ray absorption spectroscopies, the technique typically delivers strong difference signals, even for weak photoinduced changes in the electronic or molecular structure. This allows for a rapid optimization of the pump-probe experimental parameters, which include the spatiotemporal overlap of both beams on the sample, the optimal excitation fluence and wavelength, the relevant timescales, etc.