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Photoinduced Oxidation State Change in α -Fe2O3 Studied by Femtosecond Extreme Ultraviolet Spectroscopy

Photoinduced Oxidation State Change in α-Fe<sub>2</sub>O<sub>3</sub> Studied by Femtosecond Extreme Ultraviolet Spectroscopy
(A) XUV transient absorption spectra of α-Fe2O3 reveal distinct changes in shape/position after excitation at 400 nm. (B) Initial excited state spectrum and simulated transient spectra for LMCT and d-d excited states.

Scientific Achievement
The Physical Chemistry Program developed time-resolved extreme ultraviolet (XUV) spectroscopy to measure ultrafast charge-transfer processes in condensed-phase systems.

Significance and Impact
By using the elemental and oxidation-state specificities of core-level transitions, this method can be applied to study charge carrier dynamics in novel semiconductor photocatalytic and photovoltaic systems.

Summary
Core-level spectroscopy can serve as a powerful tool for studying the dynamics of particles that carry charges in systems of condense-phase matter due to its ability to distinguish elements, oxidation states and spin states. In this work, a table-top apparatus is built to study ultrafast photoinduced phenomena via transient extreme ultraviolet (XUV) absorption technique. The material of interest, α-Fe2O3 thin-film, is excited by 400nm light; then XUV absorption spectroscopy reveals distinct changes in shape and position of the absorption peak due to charge transfer from O to Fe.

The electronic distribution changes around Fe (iron) atoms are probed by XUV photons near the 57 eV Fe M-edge. The ‘edge’ refers to the core electron that is excited, with principal quantum number 3 corresponding to the M-edge; i.e., 3p-to-3d transitions.

Since the valence band has contributions from both oxygen 2p and filled iron 3d orbitals, two bandgap excitations pathways are available. Immediately after photoexcitation, an excited state appears. The excited state’s spectral shape matches the ligand-to-metal-charge transfer state spectrum as calculated by charge transfer multiplet codes. This state then undergoes a ~240 femtosecond relaxation to a long-lived state that lasts for hundreds of picoseconds.

This work provides a novel way to investigate the concepts of charge carrier dynamics in photovoltaic and photocatalytic devices.

J. Vura-Weis, C.-M. Jiang, C. Liu, H. Gao, J. M. Lucas, F. de Groot, P. Yang, A. P. Alivisatos, and S. R. Leone, Femtosecond M2,3-edge spectroscopy of transition metal oxides: photoinduced oxidation state change in α-Fe2O3, J. Phys.Chem. Lett. 4, 3667 (2013).