The ability to perform tabletop transient absorption experiments in the soft x-ray region sits at the forefront of spectroscopic methods capable of elucidating charge transfer mechanisms in catalytically-active solid-state materials. The inherent value of soft x-ray spectroscopy to catalysis arises, not only due to the greater absorption cross-section of core-to-valence transitions relative to hard x-ray transitions, but also because of its elemental, oxidation-state, and electronic spin-state sensitivity. In order to fully utilize the benefits of this powerful technique in catalysis, experimental methods must be developed which achieve surface sensitivity, as it is the surface states of the solid that are responsible for facilitating chemical transformations. To this end, we have developed grazing-angle transient x-ray reflectivity within the 30 – 72 eV spectral window as a spectroscopic probe of such states. Since reflectivity spectra not only probe the imaginary part of the refractive index (i.e., absorption), but also the real part, it is paramount to develop theoretical methods to interconvert between transmission-absorption and reflection-absorption spectra. Using single crystal and polycrystalline α-Fe2O3, an earth-abundant heterogeneous catalyst, we clearly demonstrate that transient x-ray reflectivity at grazing angle is indeed a surface-sensitive probe of condensed materials. We find that it is the real part of the refractive index that reports on resonant transitions associated with the surface while the imaginary part is sensitive to resonances within the near surface region. This level of insight from each separable part of the refractive index allows us to elucidate the mechanism of charge transfer within each material and determine how charge carriers migrate relative to the surface of α-Fe2O3.

Achieving surface sensitivity in soft x-ray spectroscopy: Transient reflectivity of charge transfer dynamics in iron oxide

253rd National Meeting (2017)

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