Article pubs.acs.org/JPCC
Achieving Surface Sensitivity in Ultrafast XUV Spectroscopy: M2,3Edge Reflection−Absorption of Transition Metal Oxides Anthony Cirri, Jakub Husek, Somnath Biswas, and L. Robert Baker* Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States S Supporting Information *
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. Because RA spectra probe both the real (i.e., reflection) and the imaginary (i.e., attenuation) parts of the refractive index, a general method is required to analyze RA spectra. Using semiempirical 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.
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INTRODUCTION Spectroscopic methods for investigating the electronic structure and charge transfer dynamics of materials provide fundamental insight for their development and integration into a wide-range of applications, including catalysis,1 photovoltaics,2 fuel cells,3 and quantum computers.4 In particular, extreme ultraviolet (XUV) spectroscopy, which probes the M2,3-edge (3p → 3d) of first row transition metals, is well-suited for such investigations because it is element specific, sensitive to oxidation state, spinstate, and coordination geometry of the metal center, and these experiments can be performed in the laboratory with femtosecond time resolution.5−10 Despite these advantages, the majority of studies performed with XUV spectroscopy are conducted as transmission experiments, which offer two major disadvantages. First, the efficacy of many emergent technologies in catalysis, photovoltaics, batteries, and fuel cells depends on the electronic structure and charge transfer dynamics at the surface, which are known to differ significantly from bulk carrier dynamics; however, transmission experiments are not inherently surface sensitive. Additionally, high attenuation of XUV photons by the resonant absorber, the sample matrix, and the sample substrate limits the total thickness of most samples to be ≤100 nm.9 To overcome these obstacles, we have constructed a tabletop XUV spectrometer, which measures photon flux reflected from a sample surface at an 8° incidence angle relative to the surface plane. Working at this incidence angle, the total probe depth of a reflection experiment is measured to be ∼3 nm, showing that the surface specificity of this technique is similar to traditional X-ray photoemission spectroscopy (see the Supporting Information for details). Additionally, XUV reflection− © XXXX American Chemical Society
absorption (RA) spectroscopy with a high harmonic generation (HHG) light source also offers the ability to follow photoinduced surface carrier dynamics with ultrafast time resolution. In addition to inherent surface sensitivity and ultrafast time resolution, other benefits of this spectroscopy are that it is not limited by the sample thickness, and measurements can be made on functional materials regardless of sample substrate or deposition method. Herein, we show experimental XUV RA spectra for α-Fe2O3, CuFeO2, TiO2, Cr2O3, NiO, and a NiOx/ Fe2O3 heterojunction, and present a general approach to simulating the experimental RA spectra. We compare the XUV RA spectra of both single-crystal (SC) α-Fe2O3 (0001) and polycrystalline (PC) α-Fe2O3 (
