## 3 Chemistry Applications under Ambient Conditions
### Femtosecond X-ray Emission Capabilities  
The Johann-type and the von Hamos X-ray emission spectrometers of the FXE instrument can be operated simultaneously. Due to naturally higher solid angle coverage per unit of the emission energy, the scanning Johann spectrometer is more suited for applications with low signal yield (e.g., extremely low concentration, valence-to-core emission or X-ray Raman spectroscopy) or when high energy resolution is required in resonant inelastic X-ray scattering (RIXS) or high energy-resolved fluorescence detection (HERFD) type of measurements. The von Hamos dispersive-type spectrometer is adequate for comparably strong emission signals as well as when simultaneous coverage of multiple emission lines from one or several elements is required. Since the von Hamos spectrometer is operated with a single-module Jungfrau 500k detector with circa \(38\times 76\) cm\({}^{2}\) active area, loading the 16-crystall \(4\times 4\)array with different cylindrical analyzers (compatible in terms of the Bragg angle) makes it possible to concurrently measure several emission lines. Such an arrangement brings the advantage of intrinsically correlated and normalized spectra in each detector image and also removing the jitter- and drift-related uncertainty of the laser and X-ray relative pulse arrival time. As the proof-of-principle for femtosecond XES measurements at FXE, we studied photoexcited aqueous [Fe(bpy)\({}_{3}\)]\({}^{2+}\) (concentration 15 mM). This molecular model system represents a prototypical example of an iron spin transition complex and has been extensively studied in the past both at synchrotrons [4; 16; 18; 19; 51] and XFELs [22; 24; 26; 47; 4]. The photochemical dynamics of [Fe(bpy)\({}_{3}\)]\({}^{2+}\) are well known and comprise major electronic and structural changes spanning timescales from tens of femtoseconds to several nanoseconds. In short, upon visible light excitation into the manifold of the metal-to-ligand charge-transfer states (\({}^{1,3}\)MLCT) the complex relaxes to the lowest excited high spin state (quintet, spin S = 2) via intermediate spin states (triplet, spin S = 1) within a few hundreds of femtoseconds. The typical lifetime of the high spin quintet state in room temperature water solution is about 660 ps. Due to the population of anti-bonding 3d molecular orbitals in the triplet and quintet metal centered states, the complex undergoes dramatic structural changes which elongate the Fe to ligand bond distance by about 0.1 A and 0.2 A in the triplet and quintet states, respectively. This structural change makes both time-resolved K-line XES [18; 19; 22] and WAXS [24; 26; 51] techniques very sensitive to these intermediate steps towards the quintet state. In the XES experiment at FXE, the solution was excited by femtosecond pulses (50 fs FWHM duration) of 400 nm light and probed by X-ray pulses at 9.3 keV. Both the laser and the X-ray pulses had a 564 kHz intra-train repetition rate (135 pulses/train) and pulse energies of 2 \(\upmu\)J and 1.4 mJ correspondingly. The X-ray trains arrived with a frequency of 10 Hz, while the laser trains were adjusted to a 5 Hz delivery rate, thus subsequent X-ray trains probed the sample at alternating laser ON (\"pumped\") and laser OFF (\"unpumped\") conditions. The solution was flowed in a 100 \(\upmu\)m round jet with a constant speed of about 60 m/s. Given the X-ray and the laser round spots of 10 \(\upmu\)m and 70 \(\upmu\)m (FWHM diameters), respectively, and the intra-train repetition rate of this experiment at 564 kHz, the solution was refreshed for each pump-probe event, which was additionally visually confirmed using a triggered sample microscope. The iron K\(\alpha\) and K\(\beta\) emission lines were simultaneously recorded on a Jungfrau 500k detector for several pump-probe delays (each delay point with about 30 s exposure); a typical detector image is shown in Figure 3A.