Extra-atomic Relaxation Energy Research Articles

ChemInform Abstract: The Wagner Plot and the Auger Parameter as Tools to Separate Initial‐ and Final‐State Contributions in X‐Ray Photoemission Spectroscopy

AbstractReview: 152 refs.

The Wagner plot and the Auger parameter as tools to separate initial- and final-state contributions in X-ray photoemission spectroscopy

The application of the Wagner plot and Auger parameter concepts as tools for the identification of chemical states of atoms in molecules or solids is growing steadily in the surface science literature. In several publications, however, the chemical and physical insights provided by such conceptual tools are not fully investigated. In this contribution it is shown how easy is to use the Wagner plot to get information on the local valence charge and Madelung (molecular) potential (initial-state effects) and on the extra-atomic relaxation energy gained from the environment (final-state effects) for a core-ionized atom. To cover the gap with previous reviews on the applications of the Wagner plot and the Auger parameter concepts, a brief overview of the contributions which appeared in the literature in the last 15years is presented.

Barium adsorption on the chemisorbedO(2 × 1)/Ni(110) surface

Barium adsorption on the O(2 × 1)/Ni(110) surface has been studied by Auger electron spectroscopy and work function measurementsin combination with photoemission measurements. The study was focused on the lowcoverage regime from submonolayer to double monolayer. The results show that duringdevelopment of the first layer of Ba on the surface, a two-dimensional incompletebarium oxide layer, BaO, forms. This BaO layer is interspersed by Ba chemisorbedatoms reacting directly with Ni atoms. As the second layer of Ba is completed, theadsorbate approaches the metallic phase due to the Ba–Ba interaction. The lowenergy Auger transition lines of Ba (75 eV) and BaO (68 eV) shift towards lowerenergies as the Ba coverage increases. Previous photoemission measurements bysynchrotron radiation are used to interpret these energy shifts, which are closelyrelated to the barium oxidation process on the surface. The analysis shows theimportance of the extra-atomic relaxation energy due to (1) the polarization of theO2− anions from BaO and (2) the screening from the electron density at the Fermi level of thebarium overlayer and the nickel substrate.

Auger and photoelectron relaxation energy in aluminum compounds: A cluster model

The energies of the Al Auger KL 2,3L 2,3 transitions and the Al 2p photoelectron peak have been determined experimentally by AES, XPS and XAES for several Al compounds. The experimental results are compared with ab initio calculations based on a cluster model in which a central aluminum atom is surrounded by its first neighbours in the crystal. This model includes chemical, structural and relaxation effects and it allows the direct ab initio determination of the intra- and extra-atomic relaxation energies. These values are compared with the Wagner equation that relates the extra-atomic relaxation energy and the Auger parameter. The three approximations that support this equation are examined on the basis of the cluster results.

Barium and oxygen interaction on the Ni(110) surface at low coverages studied by softx-ray photoemission spectroscopy: Ba negative binding energy shifts and their correlationwith Auger electron spectroscopy shifts

In this work, barium and oxygen interaction on the Ni(110) surface is investigated, bymeans of soft x-ray photoemission spectroscopy (SXPS), mainly at submonolayer andmonolayer coverages. The results show that oxygen interacts with barium and the substrateas well forming BaO and NiO respectively. The formation of both oxides is consistent withprevious Auger electron spectroscopy (AES) results. The oxidation of barium on thesurface induces negative binding energy shifts of the Ba low core atomic levels 4d, 5s and5p. Both initial and final state effects are shown to be necessary in order to explainthese peculiar energy shifts. This interpretation is based on correlating the corelevel binding energy shifts with previously recorded AES transition line shifts,also taking into account the changes of the work function of the surface. Theanalysis shows that the extra-atomic relaxation energy decreases on going frommetallic Ba to BaO. This implies that the reduction of the free electron screeningmore than counteracts the increase in relaxation energy due to the polarizableO2− anions.

Preparation and characterization of NaInO2 and NaInS2

NaInO2 and NaInS2 with α-NaFeO2 layer type structure have been prepared and characterized by X-ray photoelectron spectroscopy to clarify the role of the anions in the electronic structures of the compounds. The In 3d5/2 binding energy for both compounds was the same, though the Na 1s binding energy of NaInO2 was larger than that of NaInS2 by 0.4 eV. The change in the Na Auger parameter, defined as the sum of Na 1s binding energy and KLL Auger kinetic energy, suggests that the difference in extra-atomic relaxation energy between the compounds causes the Na 1s binding energy shift. Discrete variational-Xα model cluster calculations for NaInO2 and NaInS2 mirrored the experimental valence band spectra and the difference of 0.5 eV in optical absorption energy of the compounds. The cluster calculation revealed that in NaInO2 the highest occupied level is formed by an antibonding Na–O bond, whereas that in NaInS2 is formed by a bonding Na–S bond. The covalent Na–S bond decreases the extra-atomic relaxation energy for Na 1s electron emission from NaInS2.

Extra‐atomic relaxation energies and auger parameters of titanium compounds

A series of titanium compounds have been studied by x-ray photoelectron spectroscopy within the Auger parameter formalism. Two different titanium Auger parameters are reported, both calculated from binding energies resulting from photoemission from Ti 2p3/2 core-level states. Of the two values, one has been commonly used for titanium Auger parameter analysis and involves a valence-level Auger transition (L3M2,3Mv), whereas the other is based purely on core-level transitions (L3M2,3M2,3). Both parameters correlate roughly with each other and with extra-atomic relaxation energies calculated from surrounding nearest-neighbor anion polarizabilities. However, the core-level-based Auger parameter shows significantly better correlation with predicted relaxation energy values, particularly when corrections are made for M2,3 multiplet splittings in the analysis. The ability to model extra-atomic relaxation with polarization energies suggests that the ligand polarizabilities in titanium thin-film and powder samples may be estimated reliably from Auger parameter data despite the high degree of covalent character found in many of these compounds. Copyright © 1999 John Wiley & Sons, Ltd.

Extra-atomic relaxation energies and auger parameters of titanium compounds

A series of titanium compounds have been studied by x-ray photoelectron spectroscopy within the Auger parameter formalism. Two different titanium Auger parameters are reported, both calculated from binding energies resulting from photoemission from Ti 2p3/2 core-level states. Of the two values, one has been commonly used for titanium Auger parameter analysis and involves a valence-level Auger transition (L3M2,3Mv), whereas the other is based purely on core-level transitions (L3M2,3M2,3). Both parameters correlate roughly with each other and with extra-atomic relaxation energies calculated from surrounding nearest-neighbor anion polarizabilities. However, the core-level-based Auger parameter shows significantly better correlation with predicted relaxation energy values, particularly when corrections are made for M2,3 multiplet splittings in the analysis. The ability to model extra-atomic relaxation with polarization energies suggests that the ligand polarizabilities in titanium thin-film and powder samples may be estimated reliably from Auger parameter data despite the high degree of covalent character found in many of these compounds. Copyright © 1999 John Wiley & Sons, Ltd.

A Combined X-Ray Photoelectron and Auger Electron Spectroscopic Study of Cesium in Variable-Charge Montmorillonites

A combined X-ray photoelectron and Auger electron spectroscopic investigation of sorption and characterization of bonding states of Cs in montmorillonite (M) and in four reduced-charge montmorillonites (RCMs) were carried out. The nondestructive analysis of sorption of Cs in variable-charge montmorillonites obtained by XPS is consistent within 4–8% to those obtained by solution analysis using AAS. The core-level Cs 3d5/2 photoelectron binding energy was observed to be enhanced with the reduction of electron population from higher to lower charge montmorillonite. The charge potential model has been used to interpret such core-level short-range chemical shifts. However, a larger shift in the Auger Cs–M4N4,5N4,5 line was observed and Cs-Auger kinetic energy was found to decrease from higher charge RCM to lower charge RCM, which may be attributed to the effect of extra-atomic relaxation from surrounding oxygen atoms; this effect implies that the flow of electronic charge to the Cs ion is larger for higher charge RCM than for lower charge RCM. However, this effect is enhanced by the extra contribution of Li that has migrated. A comparison of Cs 3d binding energies and Cs-Auger kinetic energies in a two-dimensional chemical state plot indicates multiple bonding states of Cs in variable-charge montmorillonites. The extra-atomic relaxation energy (ΔEr) value of each of the Cs–montmorillonites was unique, which indicates that this is dependent on the chemical environment and on the availability of electron density in montmorillonites. Also, the binding energy value of Cs 3d5/2 was found to be nearer to CsOH than to CsCl, which reflects that the bonding state of Cs is not as ionic as CsCl.

Extra-atomic Relaxation and Core-Level Binding Energy Shifts at Silicon/Silicon Oxide Interfaces: Effects of Cluster Size on Physical Models

The Si 2p and O 1s core-level photoemission peaks and the valence band peaks for silicon oxide films grown on crystalline silicon have been observed to shift to higher binding energy as a function of increasing film thickness. The physical basis for this shift has been ascribed to a variety of mechanisms including initial state effects, final state effects, and extrinsic effects. By constructing a structurally homogeneous silicon oxide film consisting of H10Si10O15 on silicon, initial state effects have been minimized and the magnitude of final state effects directly measured. Unlike previous studies utilizing the H8Si8O12 cluster, the cluster precursor utilized in these studies is large enough to allow direct observation of the theoretically predicted asymptotic behavior in the core-level shift as a function of cluster layer thickness prior to the onset of film charging.

Energy correction by evaporated gold in x-ray photoelectron spectroscopy

This work reports measurements of the binding-energy shift of gold evaporated on several insulating materials including polymers such as polytetrafluoroethylene and polyethylene and inorganic materials such as silicon oxide. The Au binding energy showed higher values than that for bulk gold in the initial stage of deposition even after correction for charging effects. The energy shifts seen for the polymer substrates were smaller to those for inorganic materials such as silicon oxide and silicon nitride. The thickness of gold necessary to obtain the bulk Au binding energy was small for substrates whose Au binding-energy shifts were small. Thus, it was confirmed that it is very important to deposit an appropriate amount of gold to obtain the correct binding energies. These results can be explained by extra-atomic relaxation, because a larger extra-atomic relaxation energy is expected for substrates on which gold atoms or clusters can migrate easily and form larger clusters. These clusters cause smaller binding-energy shifts and require smaller amounts of evaporated gold.

The extra-atomic relaxation and energy relations on the adsorbate-substrate interface

Energy relations important for the photoionization processes on the adsorbate-substrate interface are obtained with the help of smooth matching between the image potential outside the substrate and the exchange-correlation potential in the substrate bulk. Using these relations the extra-atomic relaxation for the electron levels of physisorbed atoms and molecules can be represented in a simple analytical form. It is then possible to obtain a general equation C − Δφ = E b F + φ, where E b F is the binding energy with respect to the Fermi level, φ is the work function of the clean substrate and Δφ is the change of the work function after adsorption. The calculated C values for Xe5p, Xe3d, and σ g, π u and σ u levels of N 2 molecules equal 11.6 eV, 65.7 eV, 13.6 eV, 15.0 eV and 16.8 eV, respectively, and are in good agreement with the experimental data. A similar relation is also obtained for the physisorption on the surface steps. The PAX method is discussed in connection with these relations. The presence of the induced Friedel oscillation in the final photoionization state is shown with the help of the experimental extra-atomic relaxation values. These oscillations are important for the difference in extra-atomic relaxation energy for valence and inner levels of the same atom or molecule and connect the extra-atomic relaxations of the valence levels with the spatial distribution of the corresponding wave functions.

Semiconducting rhenium silicide thin films on Si(111)

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 Å) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x-ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 Å) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 Å) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600 °C. In the Re-Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2p and Re 4f) are the same in the compound and in the elements. As the energy shift of the Si KLL Auger is negligible also, the extra-atomic relaxation energy for Si atoms is the same in silicide as in silicon, indicating that ReSi2 has a semiconducting character. UPS results confirm this assumption: the density of states near EF decreases strongly upon ReSi2 formation and at the same time the valence band edge shifts from EF to lower binding energy. GIXD gives sharp diffraction peaks, characteristic of ReSi2 (110) in epitaxy on Si(111). This technique also reveals that the films present an additional orientation near the interface. Optical absorption measurements performed on ReSi2 films of ∼300 Å thick indicate that this silicide is a semiconductor with an indirect energy gap of 0.15 eV, in agreement with previous studies.

Extra-atomic relaxation energy calculations using an extended potential model

We show how the ground state potential model used to analyse XPS shifts can be extended to account for final state relaxation. The extended potential model is found to describe rather well extra-atomic relaxation contributions to the XPS and Auger metal-atom shifts.

Polarization and extra-atomic relaxation on the surface and in the bulk of solids: influence of the molecular shape, polarizability and complex nature of the adsorbate

The influence of the molecular cavity shape and of the angular dependence of the dielectric constants in a low symmetry crystal on the polarization energy E pol, induced by electron holes in the bulk and on the surface of the molecular solids, is investigated. The analytical expressions are obtained and evaluated for SF 6, SO 2, CS 2 and seven aromatic organic compounds. The theory is in good agreement with the experimental data. The deviation of the molecular cavity shape from a sphere has a marked influence on the calculated extra-atomic relaxation energy in large molecules. The E pol value depends on the hole position. In SF 6 this value is larger for F atom photoionization than for S atom photoionization. The angular dependence of the dielectric constants has a smaller effect on the E pol value than does that of the cavity shape. The E pol value on the surface of a single crystal depends on the crystallographic plane. The polarization energy for CO molecules on the top of Xe layers on Al(111) is calculated in dependence on the number of Xe layers. The calculated extra-atomic relaxation energy is in good agreement with experiment.

Effective atomic charges and charge transfer after photoionization in sulfur compounds and phosphorus compounds

The effective charges, extra-atomic relaxation energy and Madelung potentials are determined in some sulphur and phosphorus compounds using experimental values and calculations. The charge transfers after photoionization are calculated using a transition state model. The charge transfer is proportional to the polarizability of the neighbor atom for the atom under investigation. The extra-atomic relaxation energy for the 1s level is about 0.5–1.0 eV larger than that for 2p level. The effective atomic charges and Madelung potentials are in good agreement with existing theoretical calculations. The obtained data are checked by the independent determination of the effective atomic charges from Madelung potentials for AX n -type molecules.

Core-level electronic structures of rare-gas atoms implanted in transition metals studied by XPS and XAES

The electronic structures in core levels have been investigated for rare-gas atoms (Ne, Ar, Kr, Xe) implanted in a series of 3d transition metals. The extra-atomic relaxation energies of the core-electron emission were estimated from X-ray photoelectron spectra (XPS) and X-ray-induced Auger electron spectra (XAES). It was found that the extra-atomic relaxation energies for neon and xenon in 3d transition metals increase with the number of d electrons in the matrix metals, and the values for xenon are larger by 1–2 eV than those for xenon adsorbed on metal surfaces. It was also elucidated that the magnitudes of the relaxation energies for rare-gas atoms in the same metal matrix increase with the atomic radii of the rare-gas atoms. These findings are discussed in relation to the screening of core holes by d electrons of the matrix metals.

Core-level electronic structures of rare-gas atoms implanted in transition metals studied by XPS and XAES

Abstract The electronic structures in core levels have been investigated for rare-gas atoms (Ne, Ar, Kr, Xe) implanted in a series of 3d transition metals. The extra-atomic relaxation energies of the core-electron emission were estimated from X-ray photoelectron spectra (XPS) and X-ray-induced Auger electron spectra (XAES). It was found that the extra-atomic relaxation energies for neon and xenon in 3d transition metals increase with the number of d electrons in the matrix metals, and the values for xenon are larger by 1–2 eV than those for xenon adsorbed on metal surfaces. It was also elucidated that the magnitudes of the relaxation energies for rare-gas atoms in the same metal matrix increase with the atomic radii of the rare-gas atoms. These findings are discussed in relation to the screening of core holes by d electrons of the matrix metals.

X-ray photoelectron spectroscopy study of ultrathin oxide layers on Al and Si substrates

The initial stages of oxide growth on Al and Si are studied by X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy methods. The change from initial Si-O and Al-O bonding through interface layer formation to SiO 2 and Al 2O 3 oxide growth are characterized in terms of charge transfer, relaxation energy and particular bonding together with oxide Fermi level shifts. The relation between the Auger parameter and hole localization or extra-atomic relaxation energies is discussed, taking into account the influence of excessive oxygen atoms presented in the oxide layer. The advantages of using these substrates for the preparation of a model catalyst and for investigating the metal-oxide support interaction are discussed. While the diffusion of deposited metal prevails for oxide thickness up to 1.2 nm, charge transfer and tunneling processes dominate in the interaction of metal deposited on thicker oxide interlayers.

In-framework ion dipole polarizabilities in non-porous and porous silicates and aluminosilicates, determined from Auger electron spectroscopy data

The in-framework ion dipole polarizability is determined by evaluating the absolute extra-atomic relaxation energy during the two-electron Auger process. A simple model has been derived by combining general relaxation theory with an electrostatic model due to Moretti (Surf. Interface Anal., 1990, 16, 159). The method is applied to crystalline MgO in order to check its validity. The result is in very good agreement with (ab initio) theoretical and experimental data. When applied to porous and non-porous silicates and aluminosilicates, the method is able to distinguish between different types of in-framework oxygens according to their site symmetry and the nature of the cation (Si, Al).