PNNL

Abstract

We report valence-to-core (VTC) X-ray emission spectroscopy (XES) measurements of the electrochemical sequence e-VOPO4, e-LiVOPO4, e-Li2VOPO4 and the reference oxides V2O3, VO2, and V2O5. Our results demonstrate laboratory-based X-ray spectroscopy instrumentation is a viable route for attaining well-resolved VTC-XES features, even for samples of limited quantity or suffering from sensitivity to the atmosphere. Moreover, an extremely efficient use of flux in the present configuration permitted these results to be captured within several hours. Thus, this study represents a framework for interrogating the molecular bonding structure of a wide range of systems with a technique which has garnered substantial interest in the subfield of catalysis employing metalloenzymes. The experimental results are in good agreement with results produced by real-space Green’s function and time-dependent density functional theory (TDDFT) methods. In particular, the latter reproduces spectral features with high fidelity and at fluorescence energies which are consistent between each set of calculations. Furthermore, the TDDFT framework lends itself naturally to investigating the character of the molecular orbitals involved in each transition. In particular, the distribution of atomic orbitals contributing to spectral features is determined. Moreover, VTC-XES is inherently sensitive to extreme local changes in electronic structure. For that reason, the projection of the dipole transition matrix along the direction of each unique ligand group is investigated and proposed as a useful way to interrogate the component of the VTC-XES which is sensitive to a given class of ligand.