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Nelin, Connie J.

Publications and source records attributed to Nelin, Connie J..

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

The XPS of Azines: A Comparative Study

A detailed analysis is presented of the X-ray Photoelectron Spectroscopy, XPS, of thin films of three Azines: Pyrazine, Pyridine and Pyrimidine. This includes not only the binding energies of the various core ionizations but also their intensities. A major focus is to compare our theoretical predictions with our measured XPS for N(1s) and C(1s) as a basis for assigning the features and for justifying the broadening parameters that must be applied to the theoretical results. The C(1s) XPS of Pyridine and Pyrimidine are significantly broadened because of unresolved XPS for their inequivalent C atoms. The extent of the binding energy, BE, shifts and the XPS intensities for the unique C atoms, which are responsible for this broadening, are obtained from the theory. The additional broadening parameters to be applied to the theory to enable comparison with measured XPS are discussed in terms of lifetime broadening, experimental resolution, and BE shifts in different layers of the thin film. A novel feature of our analysis is that it is necessary to invoke surface core level shifts to explain and to justify the broadenings needed to apply to the theoretical results to have a complete comparison with the measured XPS. The results presented have general value for extracting chemical and physical information from XPS.

Bagus, Paul↗
DOE OSTI2024

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

For this work, 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. Then, advanced methods are used to explain and contrast the properties of different groups of ionic states. An important conclusion is that in most cases, the ionic states cannot be described with a single configuration or determinant. Despite this essential many-body character of the XPS, we demonstrate that it is possible to understand the origin of the main and satellite XPS features in terms of their orbital character.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

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

The origin and assignment of the complex main and satellite XPS features of the cations in ionic compounds has 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. However, there are strong disagreements about the assignment of XPS features to these configurations. The present work is directed toward resolving the origin of main and satellite features for the Ni 2p XPS of NiO based on ab initio molecular orbital wavefunctions for a cluster model of NiO. A major problem in earlier ab initio XPS studies of ionic compounds has been the use of a common set of orbitals that was not able to properly describe all the ionic configurations that contribute to the full XPS spectra. This is resolved in the present work by using orbitals that are optimized for averages of the occupations of the different configurations that contribute to the XPS. The approach of using State Averaged orbitals is validated through comparisons between different averages and through use of higher order excitations in the wavefunctions for the ionic states. It represents a major extension of our earlier work on the main and satellite features of the Fe 2p XPS of Fe 2 O 3 and proves the reliability and the generality of the assignments of the character and origin of the different features of the XPS obtained with orbitals optimized for State Averages. These molecular orbital methods permit the characterization of the ionic states in terms of the importance of shake excitations and of the coupling of ionization of 2p 1/2 and 2p 3/2 spin-orbit split sub shells. Further, the work lays the foundation for definitive assignments of the character of main and satellite XPS features and points to their origin in the electronic structure of the material.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Many-body effects for the Mg 2s XPS of MgO

The energy separation of the higher lying cation XPS binding energies, BEs, for MgO is examined. It is shown that the Hartree–Fock BEs overestimate the separation of the Mg BE(2s) and BE(2p), because an important many-body effect is neglected in the determination of the Hartree–Fock BEs. Once the many-body effect is taken into account, there is good agreement between theory and the XPS measurement. The character of this atomic many-body effect is established in terms of a nearly degenerate configuration that is needed to describe the wavefunction when an Mg 2s electron is ionized. Further, the results provide additional evidence of the ionic character of the MgO crystal. Given the atomic character of the many-body effect, it is likely to apply also to the BE separations of other closed shell oxides.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Electronic structure of reduced CeO 2 (111) surfaces interacting with hydrogen as revealed through electron energy loss spectroscopy in comparison with theoretical investigations

Based on both, ab-initio cluster calculations as well as periodic density functional based loss function calculations, we have assigned the origins of the valence electron excitation regime in electron energy loss spectra of well-ordered ceria films in (111) orientation at various states of reduction as well as after exposing the reduced films to hydrogen from the gas phase (Li et al, Angew. Chem. Int. Ed., 58 (2019) 14686–14693). The explicit calculation of intensity distributions using the dipole approximation allow us to draw conclusions about the nature of oxygen vacancies, which occur upon reduction and how those interact with hydrogen to form hydride species. Herein, we also reports a brief discussion of vibrational excitations of hydrogen loaded ceria and corroborates previous interpretations, based on inelastic neutron scattering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Covalency in Fe 2 O 3 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) 2 O 3 . 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. It is found that the covalency is considerably different for different oxidation states and for different ionized and non-ionized configurations. The changes in covalency for the ions are shown to be responsible for important changes in relaxation energies for X-Ray Photoelectron Spectroscopy (XPS) spectra and in the intensity lost from main XPS peaks to shake satellites. While these consequences are not observables themselves, they are important for the interpretation of the XPS spectra, in particular, for efforts to extract stoichiometries of these iron oxides from XPS data. This is a finding likely applicable across various 3d transition metal oxide materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Analysis of the Fe 2p XPS for hematite α Fe2O3: 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-Fe2O3) 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. It is shown that a one-body or one-configuration analysis is not sufficient and that the many-body, many-determinantal, and many-configurational character of the wavefunctions must be taken into account to describe and understand why the XPS intensity is spread over an extremely large number of final 2p-hole multiplets. The focus is on the consequences of angular momentum coupling of the core and valence open shell electrons, the ligand field splittings of the valence shell orbitals, and the degree of covalent mixing of the Fe(3d) electrons with the O(2p) electrons. Novel theoretical methods are used to estimate the importance of these various terms. An important consequence of covalency is a reduction in the energy separation of the multiplets. Although shake satellites are not considered explicitly, the total losses of intensity from the angular momentum multiplets to shake satellites is determined and related to the covalent character of the Fe-O interaction. The losses are found to be the same for Fe 2p1/2 and 2p3/2 ionization.

Bagus, Paul S. (ORCID:0000000349315217)↗
DOE OSTI2019

Revisiting surface core-level shifts for ionic compounds

The shifts of core-level binding energies can provide powerful information about the electronic structure of a material. Understanding the physical origin of these shifts for catalytically relevant oxides may provide important insight into their properties. This requires reliable theoretical methods which are able to relate the binding energy shifts to the electronic structure. In order to establish such a methodology, the CaO(100) surface to bulk core-level binding energy shifts have been studied with Hartree-Fock and density-functional theory methods using both cluster and periodic slab models. The shifts obtained from the different theoretical methods are compared with each other and with data from synchrotron-based x-ray photoelectron spectroscopy (XPS) measurements. With a common approximation for the slab model treatment of XPS, the predicted binding energy shifts are seriously in error. In this work, the origin of the error is identified as arising from a flawed treatment of the surface atom binding energies, and a method for correcting the failure is presented.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗