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 [1].
In a recent application [2], we used femtosecond resonant inelastic x-ray scattering (RIXS) at the free-electron laser LCLS in Stanford (USA) [3] 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 [4] by revealing the local chemistry and its dynamical evolution in metalloproteins.
[1] Ph. Wernet, Electronic structure in real time: Mapping valence electron rearrangements during chemical reactions, Phys. Chem. Chem. Phys. 13, 16941 (2011).
[2] Ph. Wernet et al. Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution, Nature 520, 78 (2015).
[3] 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).
[4] 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).
