Connie J. Nelin

Analysis of the Broadening of X‐ray Photoelectron Spectroscopy Peaks for Ionic Crystals

X-ray photoelectron spectroscopy (XPS) is one of the most widely applied and powerful surface-sensitive investigative tools in heterogeneous catalysis research and yields, mainly by analyzing the area and the position of XP peaks, quantitative and element-specific information on the various stages in a catalyst s life, for example, surface speciation of precursor materials, activation, aging and poisoning, and electronic structure of the active site. Recently, strong evidence has been provided that the flexibility of the geometric arrangement of atoms in the active sites of a catalytic system strongly influences the system s activity and selectivity. This flexibility is intimately connected to the phonon structure of materials. The so-called polaronic distortion of thin films has been discussed as an important factor in thin-film oxide systems, which have recently been shown to stabilize charge transfer from the metal substrate through the oxide film to the adsorbed molecule or metal. The g-tensor properties of O2 spontaneously formed on a thin MgO(100) film on Mo(100) and investigated with ESR spectroscopy under ultra-highvacuum conditions directly revealed the polaronic distortion upon charge transfer. Another way of investigating vibrational properties is to look at the broadening in photoelectron spectra. This broadening is natural and easy to observe in Xray photoelectron spectra, which are routinely measured for oxide films on metal supports. However, there has not been a way to relate the broadening of the XPS peaks to the chemical and catalytic activity of the oxide films. For the first time we demonstrate, by comparing experiments on MgO thin films with ab initio calculations, that the changes in bond lengths between the initial ground state and the core-hole ionized states are directly related to the extent of the vibrational broadening. It is reasonable to expect that changes in the initial-state polaronic distortion will significantly affect the final-state changes in bond length. In this context, it is also important to recall that charge-transfer processes have been postulated and shown to affect the catalytic properties of thinfilm oxide systems. In this work on the Franck–Condon vibrational broadening of the XPS peaks of MgO, we have compared the extreme charging cases of core holes on an Mg cation, which is doubly positively charged, and on an O anion, which is doubly negatively charged. It has been known for some time that vibrational excitations lead to broadening, and sometimes even distinct structure, in XPS and photoabsorption spectroscopies (XAS). This broadening is due to the change in the equilibrium nuclear geometry between the initial ground state of a system and the final excited or ionized state. The broadening is commonly referred to as Franck–Condon (FC) broadening. Vibrational structure and FC broadening have been observed for isolated molecules as well as for adsorbed molecules and condensed systems, especially ionic crystals. The theoretical treatments that have been used for molecular spectra follow, in large measure, the work of Cederbaum and Domcke. For the XPS of ionic crystals, the theory is based on methods developed for optical excitations in solids. This theory depends on parameters for the dielectric constants and the longitudinal optical modes. Unfortunately, neither of these approaches gives insight into the chemical changes and chemical bonding that lead to the observed FC broadenings. Herein, we present a new conceptual approach that focuses on the chemical interactions and that provides the foundation for understanding chemical effects that account for potential changes in the broadenings between bulk, surface, and thin films of oxides and other ionic materials. We also present new experimental results that demonstrate that such changes do indeed exist. Our work now makes it possible to use the FC broadening as a feature that will allow us to gain new qualitative insight and, hence, greater understanding of materials properties and processes relevant for catalysis. We consider specifically a theoretical formulation for cubic oxides, for example, MgO and MnO, although the extension to other geometries is straightforward. We first discuss the FC broadening in bulk MgO and then extend the model to a monolayer MgO film supported by [*] Prof. Dr. P. S. Bagus Department of Chemistry, University of North Texas Denton, TX 76203 (USA) Fax: (+1)940-565-4813 E-mail: bagus@unt.edu

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. As is expected, the covalent mixing is larger when the nominal oxidation state is higher. More subtly, the importance of the nf covalent mixing decreases sharply relative to (n + 1)d as the nf occupation increases. Atomic properties of the metal cations that drive these trends are identified.

The effect of symmetry on the U L3 NEXAFS of octahedral coordinated uranium(vi)

We describe a detailed theoretical analysis of how distortions from ideal cubic or Oh symmetry to tetrahedral, D4h, symmetry affect the shape, in particular the width, of the U L3-edge NEXAFS for U(vi) in octahedral coordination. The full-width-half-maximum (FWHM) of the L3-edge white line decreases with increasing distortion from Oh symmetry. In particular, the FWHM of the white line narrows whether the tetragonal distortion is to compression or to extension. The origin of this decrease in the FWHM is analyzed in terms of the electronic structure of the excited levels arising from the unoccupied U(6d). The relative importance of ligand field and of spin-orbit effects is examined, where the dominant role of ligand field effects is established. Especially at higher distortions, the ligand splittings decrease rapidly and lead to an accelerated, quadratic decrease in the FWHM with increasing distortion. This is related to the increase of covalent character in the appropriate component of the Oh derived eg orbitals. Our ab initio theory uses relativistic wavefunctions for cluster models of the structures; empirical or semi-empirical parameters were not used to adjust prediction to experiment. A major advantage is that it provides a transparent approach for determining how the character and extent of the covalent mixing of the relevant U and O orbitals affect the U L3-edge white line.

Consequences of realistic embedding for the L2,3 edge XAS of α-Fe2O3

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 L2,3 edge XAS of α-Fe2O3, an example of a high spin ionic crystal, on increasingly realistic models of the condensed system. It is shown that an FeO6 cluster model possessing the appropriate local site symmetry describes most features of the XAS and is a major improvement over the isolated Fe3+ cation. In contrast, replacing next nearest neighbor positive point charges with Sc3+, a closed shell cation of similar spatial extent to Fe3+, only marginally improves the match to experiment. This work suggests that second nearest neighbor effects are negligible. Rather, major improvements to the predicted L2,3 edge XAS likely requires additional many body effects that go beyond the present study in which the multiplets are restricted to arise from angular momentum coupling within a single open shell configuration.

Extracting Chemical Information from XPS Spectra: A Perspective

AbstractImportant mechanisms that lead to features, often complex, in X-ray photoelectron spectroscopy (XPS) spectra are defined and described. It is shown that there is much information in an XPS spectrum that can be obtained by examining these features rather than examining only the shifts of main peaks between different materials. These mechanisms are presented with a focus on describing the underlying chemical and physical phenomena responsible for features of the XPS and on showing how these XPS features can be related to the properties and electronic structure of the material studied. While it is necessary to consider certain quantum mechanical rules, the mathematical formalism is not discussed. However, a general awareness of multiplet splittings, which are a result of angular momentum coupling combined with ligand field and spin–orbit splittings, and of covalent mixings in the metal–ligand bond of oxides is essential to properly interpret the significance of XPS features. A conceptual framework of shake excitation from bonding to anti-bonding orbitals is introduced to provide an understanding of the significance of XPS satellites. While the coupling of theory and measurement is required to extract quantitative information from XPS, it may be possible to obtain useful qualitative information directly from features of the XPS spectra provided that one takes into account more than only shifts of the XPS binding energies.Graphical AbstractA correct analysis of XPS features may require a careful treatment of many-body effects that distribute intensity over many individual, unresolved final states.

Analysis of X-ray adsorption edges: L2,3 edge of FeCl4−

We describe a detailed analysis of the features of the X-ray adsorption spectra at the Fe L2,3 edge of FeCl4-. 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. We also explicitly take account of the intermediate coupling of the open core and valence shell electrons. Our analysis also includes comparison of theory and experiment for the Fe L2,3 edge and comparison of theoretical predictions for the Fe3+ cation and FeCl4-. The electronic structure is obtained from theoretical wavefunctions for the ground and excited states.