Multiplet XPS analysis of the Mn 2for MnOthin films.

In this work, we performed a detailed analysis of the x-ray photoemission spectroscopy (XPS) of the Mn 2peak for MnO(001) thin films. This is a challenging task since MnOis composed of two different cations, Mnat tetrahedral and Mnat octahedral sites, which both contribute to the XPS spectra. The oxide spectra consist of many multiplets arising from the angular momentum coupling of the open Mn 2and 3shells, thus increasing the spectrums' complexity.

Chemical information from XPS: Theory and experiment for Ni(OH)2.

The features and the electronic character of the states for the Ni 2p x-ray photoelectron spectroscopy (XPS) of Ni(OH)2 were analyzed. This detailed analysis is based on ab initio molecular orbital wavefunctions for a cluster model of Ni(OH)2. The theory is validated by comparison with experiment.

Bonding and Interactions in UO for Ground and Core Excited States: Extracting Chemistry from Molecular Orbital Calculations.

Theoretical analyses of actinyls are necessary in order to understand and correctly interpret the chemical and physical properties of these molecules. Here, wave functions of Uranyl, UO, are considered for the ground state and for the core excited states where an electron is promoted from the U 3d shell into a low-lying unoccupied orbital that is U 5f antibonding with the ligand, O, orbitals. A focus is on the application of novel theoretical methods to the analysis of these wave functions so that measurements, especially with X-ray absorption, can be related to the UO chemical bonding.

Actinyl Electronic Structure Probed by XAS: The Role of Many-Body Effects.

A detailed analysis of the wave functions for the M to 5f excitations in the linear actinyls, UO, NpO, and PuO, and the theoretical X-ray absorption spectra obtained with these wave functions in comparison with experimental M-edge high-resolution X-ray absorption near-edge structure (HR-XANES) spectra is presented. The wave functions include full treatment of scalar and spin-orbit relativistic effects through the use of a Dirac-Coulomb Hamiltonian; many-body effects are included in determining the wave functions. The character of the excited states and of the active spaces to describe the wave functions for these states are investigated and analyzed.

Theoretical Prediction of Core-Level Binding Energies: Analysis of Unexpected Errors.

The analysis of the C(1s) and O(1s) core-level binding energies (CLBEs) of selected molecules computed by means of total energy Hartree-Fock (ΔSCF-HF) differences shows that in some cases, the calculated values for the C(1s) are larger than the experiment, which is unexpected. The origin of these unexpected errors of the Hartree-Fock ΔSCF BEs is shown to arise from static, nondynamical, electron correlation effects which are larger for the ion than for the neutral system. Once these static correlation effects are included by using complete active space self-consistent field (CASSCF) wave functions that include internal correlation terms, the resulting ΔSCF BEs are, as expected, smaller than measured values.

Electronic Structure of Actinyls: Orbital Properties.

A detailed analysis is presented for the covalent character of the orbitals in the actinyls: UO, NpO, and PuO. Both the initial, or ground state, GS, configuration and the excited configurations where a 3d electron is excited into the open valence, nominally the 5f shell, are considered. The orbitals are determined as fully relativistic, four component Dirac-Coulomb Hartree-Fock solutions.

Vanadium oxide, vanadium oxynitride, and cobalt oxynitride as electrocatalysts for the nitrogen reduction reaction: a review of recent developments.

The electrocatalytic reduction of molecular nitrogen to ammonia-the nitrogen reduction reaction (NRR)-is of broad interest as an environmentally- and energy-friendly alternative to the Haber-Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we find that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars-van Krevelen mechanism and that the reduction of lattice nitrogen and Nto NHoccurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice; and (b) that NRR and the hydrogen evolution reaction do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride.

Main and Satellite Features in the Ni 2p XPS of NiO.

The origin and assignment of the complex main and satellite X-ray photoelectron spectroscopy (XPS) features of the cations in ionic compounds have been the subject of extensive theoretical studies using different methods. There is agreement that within a molecular orbital model, one needs to take into account different types of configurations. Specifically, those where a core electron is removed, but no other configuration changes are made and those where in addition to ionization, there are also shake or charge-transfer changes to the ionic configuration.

Origin of the complex main and satellite features in Fe 2p XPS of FeO.

Although the origin and assignment of the complex XPS features of the cations in ionic compounds has been the subject of extensive theoretical work, agreement with experimental observations remains insufficient for unambiguous interpretation. This paper presents a rigorous treatment of the main and satellite features in the Fe 2p XPS of FeO. This has been possible using a unique methodology for the selection of orbitals that are used to form the ionic wavefunctions.

XPS binding energy shifts as a function of bond distances: a case study of CO.

The O(1s) and C(1s) XPS core-level binding energies. BEs, have been studied as a function of the C-O internuclear distance for a large range of distances. The BE() for both BEs show considerable variation over the distances studied which is, however, different for the O(1s) and C(1s) BEs.

Computational and Spectroscopic Tools for the Detection of Bond Covalency in Pu(IV) Materials.

Plutonium is used as a major component of new-generation nuclear fuels and of radioisotope batteries for Mars rovers, but it is also an environmental pollutant. Plutonium clearly has high technological and environmental importance, but it has an extremely complex, not well-understood electronic structure. The level of covalency of the Pu 5f valence orbitals and their role in chemical bonding are still an enigma and thus at the frontier of research in actinide science.

Combined multiplet theory and experiment for the Fe 2p and 3p XPS of FeO and FeO.

The Al K alpha, 1486.6 eV, based x-ray photoelectron spectroscopy (XPS) of Fe 2p and Fe 3p for Fe(III) in FeO and Fe(II) in FeO is compared with theoretical predictions based on ab initio wavefunctions that accurately treat the final, core-hole, multiplets. The principal objectives of this comparison are to understand the multiplet structure and to evaluate the use of both the 2p and 3p spectra in determining oxidation states.

Covalency in FeO and FeO: Consequences for XPS satellite intensity.

The covalent character of the interaction between the metal cation and the oxygen ligands has been examined for two Fe oxides with different nominal oxidation states, Fe(II)O, and Fe(III)O. The covalent character is examined for the initial, ground state configuration and for the ionic states involving the removal of a shallow core, Fe 3p, and a deep core, Fe 2p, electron. The covalency is assessed based on novel theoretical analyses of wave functions for the various cases.

Limitations of the equivalent core model for understanding core-level spectroscopies.

The equivalent core model, or the Z + 1 approximation, has been used to interpret the binding energy, BE, shifts observed in X-ray photoelectron spectroscopy, XPS; in particular to relate these shifts to their origin in the electronic structure of the system. Indeed, a recent paper has claimed that the equivalent core model provides an intuitive chemical view of XPS BE shifts. In the present paper, we present a detailed comparison of the electronic structure provided from rigorous core-hole theory and from the equivalent core model to assess the validity and the utility of the use of the equivalent core model.

Analysis of the Fe 2p XPS for hematite α FeO: Consequences of covalent bonding and orbital splittings on multiplet splittings.

The origins of the complex Fe 2p X-Ray Photoelectron Spectra (XPS) of hematite (α-FeO) are analyzed and related to the character of the bonding in this compound. This analysis provides a new and novel view of the reasons for XPS binding energies (BEs) and BE shifts, which deepens the current understanding and interpretation of the physical and chemical significance of the XPS. In particular, many-body effects are considered for the initial and the final, 2p-hole configuration wavefunctions.

Surface core level BE shifts for CaO(100): insights into physical origins.

The relationship between the electronic structure of CaO and the binding energy, BE, shifts between surface and bulk atoms is examined and the physical origins of these shifts are established. Furthermore, the contribution of covalent mixing to the interaction, including the energetic importance, is investigated and found to be small. In particular, the small shift between surface and bulk O(1s) BEs is shown to originate from changes in the polarizable charge distribution of surface O anions.

Cluster embedding of ionic systems: Point charges and extended ions.

The embedding of cluster models of oxides with point charges and with extensions of the embedding which take into account the spatial extent of the cations is examined with an emphasis on the consequences of this embedding for the relative ionization and excitation energies that are measured in core-level spectroscopies. It is found that the dependence of the electronic structure of the oxides and the relative energies of different levels depend only weakly on the embedding and that relatively simple embeddings may be sufficient to provide an adequate model for determining core-level spectra. This is different from absolute values of the ionizations which, as expected, depend strongly on the details of the extended crystal; however, relative values of binding energies, as measured in photoemission, are of greater interest than the absolute values.

Consequences of realistic embedding for the L edge XAS of α-FeO.

Cluster models of condensed systems are often used to simulate the core-level spectra obtained with X-ray Photoelectron Spectroscopy, XPS, or with X-ray Absorption Spectroscopy, XAS, especially for near edge features. The main objective of this paper is to examine the dependence of the predicted L edge XAS of α-FeO, an example of a high spin ionic crystal, on increasingly realistic models of the condensed system. It is shown that an FeO cluster model possessing the appropriate local site symmetry describes most features of the XAS and is a major improvement over the isolated Fe cation.

Analysis of X-ray adsorption edges: L edge of FeCl.

We describe a detailed analysis of the features of the X-ray adsorption spectra at the Fe L edge of FeCl. The objective of this analysis is to explain the origin of the complex features in relation to properties of the wavefunctions, especially for the excited states. These properties include spin-orbit and ligand field splittings where a novel aspect of the dipole selection rules is applied to understand the influence of these splittings on the spectra.

Quantifying small changes in uranium oxidation states using XPS of a shallow core level.

The U 4f line is commonly used to determine uranium oxidation states with X-ray photoelectron spectroscopy (XPS). In contrast, the XPS of the shallow core-levels of uranium are rarely recorded. Nonetheless, theory has shown that the U 5d (and 5p) multiplet structure is very sensitive to oxidation state.

First-Principles Fe L-Edge and O K-Edge XANES and XMCD Spectra for Iron Oxides.

X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) spectroscopies are tools in widespread use for providing detailed local atomic structure, oxidation state, and magnetic structure information for materials and organometallic complexes. The analysis of these spectra for transition-metal L-edges is routinely performed on the basis of ligand-field multiplet theory because one- and two-particle mean-field ab initio methods typically cannot describe the multiplet structure. Here we show that multireference configuration interaction (MRCI) calculations can satisfactorily reproduce measured XANES spectra for a range of complex iron oxide materials including hematite and magnetite.

Assessing the ability of DFT methods to describe static electron correlation effects: CO core level binding energies as a representative case.

We use a total energy difference approach to explore the ability of various density functional theory based methods in accounting for the differential effect of static electron correlation on the C(1s) and O(1s) core level binding energies (BEs) of the CO molecule. In particular, we focus on the magnitude of the errors of the computed C(1s) and O(1s) BEs and on their relative difference as compared to experiment and to previous results from explicitly correlated wave functions. Results show that the different exchange-correlation functionals studied here behave rather erratically and a considerable number of them lead to large errors in the BEs and/or the BE shifts.

Covalent bonding in heavy metal oxides.

Novel theoretical methods were used to quantify the magnitude and the energetic contributions of 4f/5f-O2p and 5d/6d-O2p interactions to covalent bonding in lanthanide and actinide oxides. Although many analyses have neglected the involvement of the frontier d orbitals, the present study shows that f and d covalencies are of comparable importance. Two trends are identified.

The effect of symmetry on the U L NEXAFS of octahedral coordinated uranium(vi).

We describe a detailed theoretical analysis of how distortions from ideal cubic or O symmetry to tetrahedral, D, symmetry affect the shape, in particular the width, of the U L-edge NEXAFS for U(vi) in octahedral coordination. The full-width-half-maximum (FWHM) of the L-edge white line decreases with increasing distortion from O symmetry. In particular, the FWHM of the white line narrows whether the tetragonal distortion is to compression or to extension.