Ultrafast soft X-ray reflection-absorption spectroscopy as a probe for following surface charge carrier dynamics in catalytically-relevant functional materials.

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.

While the ability to crystallizemetal nanoclusters has revealed their geometric structure,t he lacko fasimilarly precise measure of their electronic structure has hampered the development of synthetic design rules to precisely engineer their electronic properties.W et rackt he evolution of highlyresolved electronic absorption spectra of gold nanoclusters with precisely mass-selected chemical composition in ac ontrolled environment. Simple derivatization of the ligands yields larger spectral changes than varying the overall atomic composition of the cluster for two clusters with similar symmetry and size.T he nominally metal-localized HOMO-LUMO transition of these nanoclusters lowers in energy linearly with increasing electron donation from the exterior of the ligand shell for both cluster sizes.V ery weak surface interactions,such as binding of He or N 2 ,yield significant statedependent shifts,i dentifying states with significant interfacial character.T hese observations demonstrate ap athway for deliberate tuning of interfacial chemistry for chemical and technological applications.

The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chemistry of the AuNPs. Specifically, the conduction electron spin resonance (CESR) spectra of AuNPs of diameter 1.8-1.9 nm are sensitive to ligand exchange of hexanethiol for 4-bromothiophenol on the surface of the nanoparticle. Chemisorption of the aromatic ligand leads to a shift in the metallic electron's g-factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Analysis by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chemical nature of the ligand controls the valence band structure of AuNPs.