Influence of Adsorbate-Free Atoms on Δ-XANES Signatures

The use of differential X-ray Absorption Near Edge Spectroscopy (Δ-XANES) for analysis of site specific adsorption on metallic electrodes relies on theoretical Δ-XANES signatures for analysis of experimental Δ-XANES fingerprints. A simple model, currently used in the analysis, considers changes in X-ray absorption properties of adsorbing atoms only. This model has been extended to include changes in X-ray absorption for other atoms of the same type that remain adsorbate-free. Configurational averaging has been applied to calculate difference spectra of a Pt6 cluster with an oxygen atom adsorbed at different sites. The extended theory shows that contribution of the adsorbate-free atoms might become significant as it affects the shape profiles of the theoretical signatures. The effect, most prominent at the absorption edge energy, is interpreted in terms of change in the electronic structure of the cluster due to oxygen adsorption. In addition to model dependence of the theoretical signatures, challenges to the application of Δ-XANES to the experimentally obtained fingerprints are discussed.