CHAPTER 10. Ultrafast Photochemical Reaction Trajectories Revealed by X-ray Transient Absorption Spectroscopy Using X-ray Free Electron Laser Sources

Ultrafast excited state structural dynamics of transition metal complexes have been recently studied at X-ray free electron laser (XFEL) sources that provide X-ray pulses with extraordinary photon flux and tens of femtosecond (fs) pulse durations. In this chapter, a recent study is used as an example to demonstrate that such new light sources can help in resolving electronic configurations for initial excited states before thermalization on the time scale of 100 fs or shorter. Ultrafast X-ray absorption near edge structure (XANES) spectra at the Ni K-edge for the excited states of Ni(ii) tetramesitylporphyrin (NiTMP) before thermalization were measured at the Linac Coherent Light Source (LCLS) by X-ray transient absorption (XTA) using fs laser and X-ray pulses for the “pump-probe” approach. An excited state evolved from the Franck–Condon state is captured via the experimentally observed red-shifted 1s → 4pz feature and obeys an inverted kinetics with 1 ps rise and 0.3 ps decay time constants. Calculated XANES transitions for these excited states assign a short-lived transient signal to a transiently reduced Ni(i) (π, 3dx2−y2) center resulting from intramolecular charge transfer on a timescale that has eluded previous synchrotron studies. These combined results can be used to extract excited state structural dynamics before thermalization and to capture intermediates of potential photocatalytic significance.