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Achieving Surface Sensitivity in Ultrafast XUV Spectroscopy: M2,3-edge Reflection-Absorption of Transition Metal Oxides

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Abstract

Ultrafast extreme ultraviolet (XUV) spectroscopy is a powerful tool for probing electronic structure and charge carrier dynamics in catalytic materials because of its elemental-, oxidation-, coordination-, and electronic spin-state sensitivity. To extend the benefits of this technique to investigating charge carrier dynamics at surfaces, we have developed near grazing angle XUV reflection-absorption (RA) spectroscopy. Since RA spectra probe both the real (i.e., reflection) and imaginary (i.e., attenuation) part of the refractive index, a general method is required to analyze RA spectra. Using semi-empirical calculations, we demonstrate that XUV RA spectra of first row transition metal oxides retain the element and chemical state specificity of XUV absorption spectroscopy. We find that the imaginary part of the refractive index reports on the chemical state of the metal center, while the real part is additionally sensitive to the surface morphology of the material. To demonstrate surface specificity, we measure the probe depth of XUV RA spectroscopy at near-grazing incidence angle to be only 3 nm. We present M2,3-edge RA spectra for single-crystal Fe2O3, and polycrystalline samples of Fe2O3, CuFeO2, TiO2, Cr2O3, NiO, and a NiOx/Fe2O3 heterojunction. We show that in the case of a phase impure NiOx/Fe2O3 heterojunction we are able to detect independently both phases as well as the presence of partially reduced defect states at the surface. This was verified using the ultrafast capability inherent in HHG XUV spectroscopy, where we selectively excited the LMCT band of NiO to transiently generate a Ni+ state. Consequently, this technique is expanding ultrafast XUV spectroscopy for the study of element specific surface carrier dynamics in functional materials.

© 2017 Optical Society of America

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