Femtosecond M_2,3-edge spectroscopy of transition-metal oxides: Photoinduced oxidation state change in α-Fe₂O₃

Oxidation-state-specific dynamics at the Fe M_2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe ₂O₃ thin films after 400 nm excitation reveals distinct changes in the shape and position of the 3p → valence absorption peak at ∼57 eV due to a ligand-to-metal charge transfer from O to Fe. Semiempirical ligand field multiplet calculations of the spectra of the initial Fe ³⁺ and photoinduced Fe²⁺ state confirm this assignment and exclude the alternative d-d excitation. The Fe²⁺ state decays to a long-lived trap state in 240 fs. This work establishes the ability of time-resolved extreme ultraviolet spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.