Figure 1. X-ray absorption fingerprints of molecular transformation.

(a) Schematic light-induced excited spin-state trapping for [Fe(bpy)₃]²⁺, where the Fe (red) is bonded to six N (blue) of the bpy ligands (L). After the initial photoexcitation of the LS (t_2g⁶e_g⁰L⁰) state into an MLCT state (t_2g⁵e_g⁰L¹), the system decays towards the HS (t_2g⁴e_g²L⁰) state. (b) Schematic experimental setup with 25(5) fs RMS time resolution. The X-ray beam from a free electron laser is monochromatized by a double diamond (111) crystal and focused to ∼10 μm by means of Beryllium X-ray lenses for probing [Fe(bpy)₃]²⁺ dissolved in water. (c) Changes between the LS and HS XANES spectra. Arrows indicate photon energies for which high time resolution data have been measured. (d) The HS/LS spectra change ratio (magenta line, measured at 10 ps) and the expected ratio between MLCT and LS state (orange solid line) calculated as a +1 eV shift of the MLCT spectrum with respect to the measured LS one. The dots in (d) are the measured MLCT amplitude based on a global fit of the data (see text and Fig. 2). Both curves have been scaled to 100% conversion.