## 3 Chemistry Applications under Ambient Conditions  
Chemical Dynamics on electronically excited state potentials have been largely investigated with the advent of ultrafast laser-optical spectroscopy [40; 41; 42]. The dynamic shape of these potentials governs the molecular structure changes, reaction rate and outcome. Therefore, it is highly desired to have tools at hand, which allow us to determine the potential surface shape, especially at the so called conical intersections, where potential energy surfaces cross through each other, which determines the fate or final product state of the ongoing reaction. At these conical intersections adiabatic effects govern the transition probability [43; 44].  
Over the past few years development of hard X-ray free-electron lasers enabled extending the well-established experimental optical pump/X-ray probe methodologies into the femtosecond time domain. The field of ultrafast photochemistry in solution has witnessed a number of benchmark studies taking full advantage of the direct chemical sensitivity offered by hard X-ray absorption and emission spectroscopies as well as X-ray scattering. The femtosecond resolution at XFELs allowed for element-specific spectroscopic investigations of transient processes and intermediate short-lived states in 3d transition metal complexes.  
In addition to identifying electronic relaxation pathways towards the lowest laying high spin excited state in prototypical spin-crossover systems [22; 23; 24] and charge transfer complexes [25], vibrational wavepackets were recently observed in Fe-carbene compounds using combined XES and WAXS tools [27]. Hereby transient molecular bond formation [30; 45] and vibrational dynamics [29; 31] were observed together with ultrafast molecular environment rearrangements in the immediate solvation shell [28], indicating the ability to also include intermolecular dynamic processes. At the same time, femtosecond XAS tools have been utilized for various 3d metal complexes as a combined local and selective probe to deliver detailed information on the coupled electronic and structural changes throughout ultrafast photochemical processes [46; 47], including nuclear wavepacket dynamics [48; 49].  
At the FXE station of European XFEL, the primary idea behind the design of the instrument lies in combining the X-ray structural spectroscopic and scattering methodologies into one experiment [34]. Considering the high photon energy capabilities of the European XFEL machine and FXE's flexible setup, the instrument enables vast possibilities for femtosecond X-ray absorption and non-resonant emission spectroscopies on a wide range of complexes containing elements from vanadium to molybdenum (K-edges) and from iodine to elements with the highest atomic numbers (L-edges). Additionally, the use of X-ray photon energies above those typically accessible at other FEL sources allows us to extend the range of momentum transfer covered in solution WAXS experiments, thus simultaneously providing high spatial resolution for monitoring structural changes in the solute, the thermodynamic state of the bulk solvent [50] as well as independently delivering information on solvent-shell molecular rearrangements [51], i.e., the solvation dynamics.