K- and L-edge X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) determination of differential orbital covalency (DOC) of transition metal sites

Abstract

Continual advancements in the development of synchrotron radiation sources have resulted in X-ray based spectroscopic techniques capable of probing the electronic and structural properties of numerous systems. This review gives an overview of the application of metal K-edge and L-edge X-ray absorption spectroscopy (XAS), as well as K resonant inelastic X-ray scattering (RIXS), to the study of electronic structure in transition metal sites with emphasis on experimentally quantifying 3d orbital covalency. The specific sensitivities of K-edge XAS, L-edge XAS, and RIXS are discussed emphasizing the complementary nature of the methods. L-edge XAS and RIXS are sensitive to mixing between 3d orbitals and ligand valence orbitals, and to the differential orbital covalency (DOC), that is, the difference in the covalencies for different symmetry sets of the d orbitals. Both L-edge XAS and RIXS are highly sensitive to and enable separation of and donor bonding and back bonding contributions to bonding. Applying ligand field multiplet simulations, including charge transfer via valence bond configuration interactions, DOC can be obtained for direct comparison with density functional theory calculations and to understand chemical trends. The application of RIXS as a probe of frontier molecular orbitals in a heme enzyme demonstrates the potential of this method for the study of metal sites in highly covalent coordination sites in bioinorganic chemistry.

Reduction and Oxidation of Cu Species in Cu-Faujasites Studied by IR Spectroscopy

“…Independently on IR studies, the status of Cu sites in our Cu zeolites was followed by X-ray absorption spectroscopy (XAS) at the Cu L-edge, which is an important tool for probing the properties of copper centers in transition-metal chemistry and catalysis [ 34 , 35 , 36 ]. In particular, L-edge X-ray absorption spectroscopy (XAS) probes transition from the metal 2s 2 p 6 orbitals to 3D unoccupied electronic states that depend both on the oxidation state of the metal site and the local coordination environment [ 37 ].…”

Comparative study of the around-Fermi electronic structure of 5d metals and metal-oxides by means of high-resolution X-ray emission and absorption spectroscopies

“…This semi-empirical methodology allowed for mapping the complete electronic structure of occupied (valence band) and unoccupied (conduction band) states of 3d-based materials. RXES is a photon-in photon-out technique, in which the intensities and energies of the incoming and emitted X-rays are monitored (Sá et al, 2013;Baker et al, 2017;Vankó et al, 2006a). In particular, by tuning the incident energy around the absorption edge of the element of interest, the unoccupied states are probed by electron excitation to the intermediate atomic state.…”

Ultrafast Intersystem Crossing in Acetylacetone via Femtosecond X-ray Transient Absorption at the Carbon K-Edge

“…[21][22][23][24] Electronic transitions from localized core-levels with well-defined orbital symmetries and angular momenta are inherently element-specific and also related to the charge distribution/spin of the initial and core-excited states through transition dipole matrix elements and exchange correlation. [25][26] Thus, when combined with time-resolved detection, the evolving near-edge coreto-valence spectral features report accurately on the changes in the electronic charge distributions, oxidation states, chemical environments, and spin crossover of the photoexcited molecule with atomic site specificity. 19,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] Further, the large energy separations between the absorption edges of different elements (tens to hundreds of eV) and the encoding of the nearedge spectral region by a few predominant core-to-valence transitions offer easy spectral elucidation in comparison to valence state photoionization spectroscopies, which may produce broad and overlapping spectral signatures from multiple photoionization channels.…”