Photochemically activated molecules catalyze chemical reactions, but a molecular-level understanding of how these short-lived and reactive intermediates catalyze reactions has remained elusive. A lack of suitable probes has limited the detail of our understanding due to the difficulties related with characterizing electronic excited states.
I will discuss how time-resolved soft x-ray spectroscopy at free-electron lasers offers unique opportunities for enabling a fundamental understanding of local atomic and intermolecular interactions with a novel characterization of chemical interactions on atomic length and time scales of Ångströms and femtoseconds .
In a recent application , we used femtosecond resonant inelastic x-ray scattering (RIXS) at the free-electron laser LCLS in Stanford (USA)  to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)5 in solution. Photo-induced removal of CO generates the highly reactive 16-electron homogeneous catalyst Fe(CO)4. We used, to the best of our knowledge for the first time, “anti-Stokes RIXS” to characterize a hitherto unreported excited singlet state of Fe(CO)4. We found that this either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a pentacoordinated Fe species on a sub-picosecond time scale.
The dynamics in solution will be contrasted to the gas-phase dynamics which we studied with femtosecond x-ray photoelectron spectroscopy at the free-electron laser FLASH in Hamburg (Germany).
These examples highlight the ability of femtosecond soft x-ray spectroscopy at free-electron lasers to probe frontier-orbital interactions with atom specificity. They demonstrate the role of x-ray spectroscopy at free-electron lasers in revealing the excited-state behavior of molecules and explaining how transient molecular states govern photochemical selectivity and rate.
I will end by discussing how the currently available methodology can be extended towards probing complex biomolecules in physiological conditions  by revealing the local chemistry and its dynamical evolution in metalloproteins.
 Ph. Wernet, Electronic structure in real time: Mapping valence electron rearrangements during chemical reactions, Phys. Chem. Chem. Phys. 13, 16941 (2011).
 Ph. Wernet et al. Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution, Nature 520, 78 (2015).
 K. Kunnus et al. A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources, Rev. Sci. Instrum. 83, 123109 (2012).
 R. Mitzner et al. L-edge X-ray Absorption Spectroscopy of Dilute Systems Relevant to Metalloproteins Using a X-ray Free-Electron Laser, J. Phys. Chem. Lett. 4, 3641 (2013).