Femtosecond M2,3-Edge Spectroscopy of Transition-Metal Oxides: Photoinduced Oxidation State Change in α‐Fe2O3

Oxidation-state-specific dynamics at the Fe M2,3-edge are measured on the sub-100 fs time scale using tabletop high-harmonic extreme ultraviolet spectroscopy. Transient absorption spectroscopy of α-Fe2O3 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 and photoinduced Fe state confirm this assignment and exclude the alternative d−d excitation. The Fe 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. SECTION: Spectroscopy, Photochemistry, and Excited States T resolved X-ray absorption spectroscopy is a powerful tool for probing the electronic structure of short-lived states because of the element, oxidation state, and spin state specificity of core-to-valence transitions. With the advent of third-generation synchrotrons and free-electron lasers, photoinduced nuclear and electronic dynamics of transition-metal complexes have been studied on picosecond to femtosecond time scales. First-row transition metals are generally probed at the Kand L2,3-edges, corresponding to 1s → 3d and 2p → 3d transitions. There is far less work on the M2,3-edge, or 3p → 3d transition, due to the rarity of sources in the extreme ultraviolet (XUV) spectral region from 40 to 100 eV and the need for high-vacuum sample environments. However, timeresolved spectroscopy at this edge is attractive for three reasons. First, the large overlap between the 3p and 3d wave functions leads to an absorption cross section that is 10 times larger than the L2,3-edge and 1000 times larger than the K-edge. For solidstate samples such as transition-metal oxides, the optical and XUV cross sections are of the same order of magnitude, with similar penetration depths for pump and probe beams. Second, the Coulomb and exchange coupling between the 3d electrons and the 3p core−hole produces a multiplet peak shape that is indicative of the ligand field, oxidation state, and spin state of the metal. Finally, recent advances in high-harmonic generation (HHG) enable transient absorption spectroscopy to be reliably performed in the XUV using a tabletop laserbased source, with a photon flux that is 2 orders of magnitude higher than femtosecond “slicing” beamlines and the time resolution from the femtosecond to attosecond regimes. Tabletop XUV spectroscopy has recently been used to measure gas-phase dynamics of small molecules, dielectric switching in Si, and element-specific spin dynamics in NiFe alloys and multilayers. In this work, it is shown that M2,3-edge transient absorption spectroscopy can be used to measure photoinduced oxidation state changes in a condensed-phase sample, α-Fe2O3 (hematite), which is a stable, earth-abundant semiconductor that is the subject of intense study due to its potential as a photocatalyst for water splitting. The efficiency is hampered by low electron mobility and rapid trapping of the initial photoexcited state. The nature of this initial state is a subject of continued debate due to the complex electronic structure of this material. Band gap excitation at 2.2 eV arises from d− d transitions that are spin-allowed due to magnetic coupling between Fe atoms. However, the interpretation of the major visible-light absorption feature at 3.2 eV depends strongly on the theoretical model used. This peak was first explained as a ligand-to-metal charge transfer (LMCT) transition on the basis of self-consistent field Xα scattered wave and semiempirical atom superposition and electron delocalization (ASED) calculations. Later treatments including a recent high-level Received: September 16, 2013 Accepted: October 15, 2013 Published: October 15, 2013 Letter