Core-hole Effects Research Articles

Electronic structure of oxide fuels from experiment and first principles calculations

Energy loss spectra from a variety of cubic oxides are compared with ab-initio calculations based on the density functional plane wave method (CASTEP). In order to obtain agreement between experimental and theoretical spectra, unique material specific considerations were taken into account. The spectra were calculated using various approximations to describe core-hole effects and electron. The calculations are based on both the generalized gradient approach and the local spin density approximation when dealing with the correlation to show qualitative agreement with the sensitive oxygen K-edge spectra in ceria, zirconia, and urania. Comparison of experimental and theoretical results let us characterize the main electronic interactions responsible for both the electronic structure and the resulting EELS spectra of the compounds in question.

Calculations of electron energy loss and x-ray absorption spectra in periodic systems without a supercell

Ab initio calculations of relativistic core-level electron-energy-loss spectra and x-ray absorption spectra in periodic systems are carried out using an impurity Green’s function formalism without the need for a supercell. The approach is based on a hybrid scheme incorporating a reciprocal space calculation of the multiplescattering equations, together with a real-space calculation of the excitation spectrum with a statically screened core-hole potential. The approach accounts for core-hole effects in deep-core spectroscopies of periodic systems while circumventing the supercell convergence issues encountered with conventional band-structure codes. The approach is implemented in an extension of the real-space Green’s function code FEFF. Illustrative calculations are presented for the K edges of C diamond and Cu, and the N K edge of GaN.

X-ray magnetic circular dichroism at rare-earth L2,3 absorption edges in various Ce and Gd compounds and alloys

The X-ray magnetic circular dichroism spectra at the rare-earth L 2,3 absorption edges in CeFe 2, Ce(Co x Fe 1− x ) 2, CeRh 3B 2, GdN, GdTiGe, and GdTiSi are investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the LSDA+ U method. The origin of the L 2,3 XMCD spectra in the compounds is examined. The core-hole effect in the final states as well as the effect of the electric quadrupole E 2 and magnetic dipole M 1 transitions have been investigated. The calculated results are compared with available experimental data.

Ab initiocalculation of ELNES/XANES of BeO polymorphs

ELNES/XANES of BeO polymorphs were calculated using a plane wave pseudopotential method including the core-hole effects. A projector augmented wave method was employed to reconstruct the all-electron matrix elements. Calculated spectra of beryllium and oxygen K-edges were compared for BeO with wurtzite, zinc-blende and rocksalt structures. Characteristic features that can be used to spectroscopically characterize the different polymorphs have been provided from our theoretical analysis.

Surface and core hole effects in X-ray photoelectron spectroscopy

In XPS analysis, two effects, which significantly reduce the measured peak intensity, are usually neglected: the core hole left behind in an XPS process which causes “intrinsic” excitations and excitations as the photoelectron pass through the surface region. We have calculated these effects quantitatively for various energies, geometries, and materials. Instead of considering the two effects separately, we introduce a new parameter, namely the correction parameter for XPS or CP XPS, which takes into account both effects. We define this CP XPS as the change in probability for emission of a photoelectron caused by the presence of the surface and the core hole in comparison with the situation where the core hole is neglected and the electron travels the same distance in an infinite medium. The calculations are performed within the dielectric response theory by means of the QUEELS–XPS software determining the energy-differential inelastic electron scattering cross-sections for X-ray photoelectron spectroscopy (XPS) including surface and core hole effects. This study has been carried out for electron energies between 300 eV and 3400 eV, for angles to the surface normal between 0° and 60° and for various materials. We find that the absolute effect is a reduction by 35–45% in peak intensities but that the variation in CP XPS with material, angle and energy are < ± 10% for emission angle ≤ 60° and photoelectron energy ≤ 1500 eV. This implies that when XPS analysis is done using relative intensities, the combined effect of the surface and of the core hole is typically less than ≈ ± 10% for geometries and energies normally used in XPS. In practice, it is however difficult to determine the bare peak intensity without the intrinsic electrons because the two overlap in energy.

Numerical Study of the Effect of Verification Core Hole on the Point Bearing Capacity of Drilled Shafts

This paper presents a numerical investigation of the effect of a verification core hole on the point bearing capacity of drilled shafts installed in clay shales. The verification core extracted at the shaft tip may reduce the point bearing capacity of drilled shafts as a result of degradation of clay shales and imperfect core hole infill. Finite-element analyses were conducted using the Mohr-Coulomb model with total stress material parameters estimated from laboratory tests. A series of load-displacement curves was calculated for 1 cycle of air drying and wetting; different drying durations and different core hole conditions were considered; and the point bearing capacity was determined at 3 and 5% shaft diameter displacements. The numerical analyses indicate that the point bearing capacity of drilled shafts with a verification core hole does not decrease for most cases, and the maximum reduction merely reaches 5%. Recommendations are made to reduce the effect of the verification core extracted at the sha...

Dynamical effects in x-ray absorption spectra of graphene and monolayeredh-BN on Ni(111)

We present first-principles calculations of x-ray absorption spectra of graphene and hexagonal BN monolayer on the Ni(111) substrate. Including dynamical core-hole screening effects according to the theory of Mahan-Nozieres-de Dominics (MND) results in an overall good agreement with previously published experimental data and our new observations. This approach provides a unified first-principles description of the electronic structure and core excitations in the sp(2)-bonded materials on metal surfaces and a better insight into the dynamics of screening effects. We demonstrate in particular that the observed spectral features of graphene and hexagonal BN can be well reproduced with the MND theory, and that they are determined by a delicate balance between initial and final-state effects.

Parameter-free calculations of X-ray spectra with FEFF9

We briefly review our implementation of the real-space Green's function (RSGF) approach for calculations of X-ray spectra, focusing on recently developed parameter free models for dominant many-body effects. Although the RSGF approach has been widely used both for near edge (XANES) and extended (EXAFS) ranges, previous implementations relied on semi-phenomenological methods, e.g., the plasmon-pole model for the self-energy, the final-state rule for screened core hole effects, and the correlated Debye model for vibrational damping. Here we describe how these approximations can be replaced by efficient ab initio models including a many-pole model of the self-energy, inelastic losses and multiple-electron excitations; a linear response approach for the core hole; and a Lanczos approach for Debye-Waller effects. We also discuss the implementation of these models and software improvements within the FEFF9 code, together with a number of examples.

Experimental evidence of six-fold oxygen coordination for phosphorus and XANES calculations

Phosphorus, a group V element, has always been found so far in minerals, biological systems and synthetic compounds with an oxygen coordination number of four (i.e, PO4 groups). We demonstrate here using phosphorus K-edge XANES spectroscopy that this element can also adopt a six-fold oxygen coordination (i.e, PO6 groups). This new coordination was achieved in AlPO4 doped SiO2 stishovite synthesized at 18 GPa and 1873 K and quenched down to ambient conditions. The well-crystallized P-bearing stishovite grains (up to 100μm diameter) were embedded in the back-transformation products of high pressure form of AlPO4 matrix. They were identified by elemental mapping (μ-XRF). μ-XANES spectra collected at the Si and P K edges in the Si rich region with a very low concentration of P present striking resemblance, Si itself being characteristic of pure stishovite. We can therefore infer that phosphorus in the corresponding stishovite crystal is involved in an octahedral coordination made of six oxygen atoms. First principle XANES calculations using a plane-wave density functional formalism with core-hole effects treated in a supercell approach at the P K edge for a P atom substituting an Si one in the stishovite structure confirm this assertion. This result shows that in the lower-mantle where all silicon is six-fold coordinated, phosphorus has the crystal-chemical ability to remain incorporated into silicate structures.

Atomic resolution mapping of the excited-state electronic structure ofCu2Owith time-resolved x-ray absorption spectroscopy

We have used time-resolved soft x-ray spectroscopy to investigate the electronic structure of optically excited cuprous oxide at the $\text{O}\text{ }K$ edge and the $\text{Cu}\text{ }{L}_{3}$ edge. The 400 nm optical excitation shifts the Cu and O absorptions to lower energy, but does not change the integrated x-ray absorption significantly for either edge. The constant integrated x-ray absorption cross-section indicates that the conduction-band and valence-band edges have very similar $\text{Cu}\text{ }3d$ and $\text{O}\text{ }2p$ orbital contributions. The 2.1 eV optical band gap of ${\text{Cu}}_{2}\text{O}$ significantly exceeds the one eV shift in the $\text{Cu}\text{ }{L}_{3}$- and $\text{O}\text{ }K$-edges absorption edges induced by optical excitation, demonstrating the importance of core-hole excitonic effects and valence electron screening in the x-ray absorption process.

Theory of phonon effects on photoemission spectra

Some important phonon effects observed in core and valence level photoemission spectra are discussed on the basis of nonequilibrium Green’s function theory. This theoretical framework allows us to incorporate phonon effects, such as Debye–Waller (DW) factors, Franck–Condon (FC) factors and electron–phonon interactions in a natural way. For the description of the electron–phonon interaction the phonon part of the screened Coulomb propagator plays a crucial role. We separately discuss those effects on photoemission spectra from localized core and extended valence levels. In case of core level excitation, taking into account the core-hole effects on phonon spectra, we can relate the Franck–Condon factors to the electron–phonon interaction, which are interpreted as the intrinsic phonon losses. Phonon losses during propagation in solids can be described by loss Keldysh diagrams similar to those for the plasmon losses, which are closely related to extrinsic losses. In case of extended valence excitation we show that the phonon effects destroy the interference between photoelectron waves in high-energy region, which enhances specific features in the X-ray photoelectron diffraction as observed before. In contrast the DW factors play a very minor role in low energy (UPS) region, and we can obtain detailed information on band structures by the use of angle-resolved measurements.

Core-hole effect in the one-particle approximation revisited from density functional theory

The strength of the core-hole effect in simulations of electron energy-loss near edge structures or x-ray absorption near edge structures is investigated using ab initio calculations based on the density functional theory. Calculations were performed using the WIEN2K and FEFF codes at different edges for the following model compounds: rutile ${\text{TiO}}_{2}$, MgO, and for some transition metals (Ti, Fe, Co, Cu, and Zn). We demonstrate that although a core hole is always present in any core-level spectroscopy experiment, its effect is not always observable. To observe or not a core-hole effect in any experimental situation can, however, be predicted from the examination of the material's ground-state electronic structure, especially of the states just above the Fermi level. Indeed, it is shown that the two important criteria governing the core-hole strength at a given edge depend first on the localization and on the character of the first empty states of the material under investigation, and second on the nature of the compensating charge necessary to maintain the crystal neutrality. These criteria are expected to give a good description of the core-hole effect in the one-particle approximation. However, nothing can be inferred as for the good/bad agreement between calculations and experiments for more complex cases such as excitations of delocalized semicore states or of atomiclike multiplet structures.

Resonant X-ray spectroscopy to study K absorption pre-edges in 3d transition metal compounds

Some general aspects concerning resonant X-ray spectroscopy are discussed and an analysis of the 1s3p RIXS plane in hematite (Fe2O3) and anatase titania (TiO2) is presented. We emphasize the importance of recording a full RIXS plane as opposed to line scans. In a simple system such as hematite it is possible to assign the K absorption pre-edge features to ligand field split orbitals and non-local excitations using spin-selective spectroscopy. The core hole effect separates local and non-local excitations in titania.

First-principles calculation of oxygen K-electron energy loss near edge structure ofHfO2

Oxygen K-electron energy loss near edge structures (ELNES) of monoclinic, tetragonal, andcubic HfO2 were calculated by the first-principles full-potential augmented plane wave plus local orbitals(APW+lo) method. By considering the relativistic effect as well as the core-hole effect in thecalculation, the experimental oxygen K ELNES was successfully reproduced. The first,second, third, and fourth peaks originate from oxygen p components hybridized with Hfd-eg,d-t2g, s, and p components, respectively. It was found that the spectral differencesamong the polymorphs are mainly caused by the local structure of the Hf in thecrystal.

X-ray absorption near edge structure/electron energy loss near edge structurecalculation using the supercell orthogonalized linear combination of atomic orbitalsmethod

Over the last eight years, a large number of x-ray absorption near edge structure (XANES)and/or electron energy loss near edge structure (ELNES) spectroscopic calculations forcomplex oxides and nitrides have been performed using the supercell-OLCAO (orthogonalizedlinear combination of atomic orbitals) method, obtaining results in very good agreementwith experiments. The method takes into account the core-hole effect and includes thedipole matrix elements calculated from ab initio wavefunctions. In this paper, we describethe method in considerable detail, emphasizing the special advantages of this method forlarge complex systems. Selected results are reviewed and several hitherto unpublishedresults are also presented. These include the Y K edge of Y ions segregated to the core of aΣ31 grain boundary in alumina, O K edges of water molecules, C K edges in different types ofsingle walled carbon nanotubes, and the Co K edge in the cyanocobalamin (vitaminB12) molecule. On the basis of these results, it is argued that the interpretation of specificfeatures of the calculated XANES/ELNES edges is not simple for complex material systemsbecause of the delocalized nature of the conduction band states. The long-standing notionof the ‘fingerprinting’ technique for spectral interpretation of experimental data is nottenable. A better approach is to fully characterize the structure under study, using eithercrystalline data or accurate ab initio modeling. Comparison between calculatedXANES/ELNES spectra and available measurements enables us to ascertain the validity ofthe modeled structure. For complex crystals or structures, it is necessary to use theweighted sum of the spectra from structurally nonequivalent sites for comparison with themeasured data. Future application of the supercell-OLCAO method to complexbiomolecular systems is also discussed.

The 1s x-ray absorption pre-edge structures in transition metal oxides

We develop a general procedure to analyse the pre-edges in 1s x-ray absorption near edgestructure (XANES) of transition metal oxides and coordination complexes. Transitionmetal coordination complexes can be described from a local model with one metal ion. The1s 3d quadrupole transitions are calculated with the charge-transfer multiplet program.Tetrahedral coordination complexes have more intense pre-edge structures due to the localmixing of 3d and 4p states, implying a combination of 1s 3d quadrupole and 1s 4pdipole transitions. Divalent transition metal oxides can be described similar tocoordination complexes, but for trivalent and tetravalent oxides, additional structuresare visible in the pre-edge region due to non-local dipole transitions. The 1s 4pdipole transitions have large cross section at the 3d-band region due to the strongmetal–metal interactions, which are oxygen mediated. This yields large intensity in the3d-band region but at a different energy than the local 1s 3d quadrupole transitionsbecause of smaller core-hole effects due to the delocalization of the excited electron.

Ab initio simulation of core-hole effects on the X-ray absorption near edge structure of GaP

The core-hole effects on X-ray absorption near edge structure (XANES) of GaP with pressure have been investigated using all-electron full potential linearized augmented plane wave method. Four electronic configurations, i.e., ground state (GS), transition state (TS), final state (FS), and Z +1 approximation ( Z +1), in the treatment of core-hole have been employed. The calculated results suggest that the experimental spectra are well reproduced by the FS and Z +1 configurations. Our XANES results support the metallic GaP-II has a Cmcm symmetry.

Pre-edge XANES structure of Mn in (Ga,Mn)As from first principles

Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Cukrovarnicka 10, 162 53 Prague 6, Czech Republic * Corresponding author. Tel.: +420-220-318-459; fax: +420-233-343-184; e-mail: gonchar@fzu.cz Received February 20, 2009; accepted May 18, 2009; available on-line November 16, 2009 The X-ray absorption near-edge structure (XANES) at the Mn K-edge in the (Ga,Mn)As magnetic semiconductor was simulated using the full potential linearized augmented plane wave (FLAPW) method including the core-hole effect. The calculations were performed in the supercell scheme for a substitutional and two tetrahedral interstitial Mn sites. The resulting pre-edge absorption structures show sharp distinction between the spectra simulated for the substitutional and interstitial Mn defects, which is determined mainly by the second nearest neighbor ligands. A single feature is obtained for the substitutional Mn impurity, whereas two peaks appear for both types of interstitial defects. An interpretation of the pre-edge features is proposed. (Ga,Mn)As magnetic semiconductor / X-ray absorption near-edge structure / Substitutional / Interstitial / Defects Introduction The Mn-doped GaAs magnetic semiconductors are interesting and promising magnetic materials for potential applications in spin electronics (spintronics). The main motivation behind the preparation and investigation of these materials is the man-made combination of both semiconducting/semimetallic and ferromagnetic properties in one physical system [1-3]. Various configurations of Mn dopants affect the magnetic properties of the (Ga,Mn)As system in different ways. There are several possibilities for a Mn atom to be incorporated in GaAs, and three of them have comparable energy [4] . The substitutional Mn atoms, Mn

Experimental and theoretical study of the structural environment of magnesium in minerals and silicate glasses using X-ray absorption near-edge structure

X-ray absorption spectroscopy at the Mg K-edge is used to obtain information on magnesium environment in minerals, silicate and alumino-silicate glasses. First-principles XANES calculations are performed for minerals using a plane-wave density functional formalism with core-hole effects treated in a supercell approach. The good agreement obtained between experimental and theoretical spectra provides useful information to interpret the spectral features. With the help of calculation, the position of the first peak of XANES spectra is related to both coordination and polyhedron distortion changes. In alumino-silicate glasses, magnesium is found to be mainly 5-fold coordinated to oxygen whatever the aluminum saturation index value. In silicate glasses, magnesium coordination increases from 4 in Cs-, Rb- and K-bearing glasses to 5 in Na- and Li-bearing glasses but remains equal as the polymerization degree of the glass varies. The variation of the C feature (position and intensity) is strongly related to the alkali type providing information on the medium range order.

X-ray magnetic circular dichroism inCeFe2: First-principles calculations

The x-ray magnetic circular dichroism (XMCD) spectra of ${\text{CeFe}}_{2}$ at the $\text{Ce}\text{ }{L}_{2,3}$, ${M}_{4,5}$, $\text{Fe}\text{ }K$, and ${L}_{2,3}$ edges are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement between the theory and the experiment at the $\text{Ce}\text{ }{M}_{5}$ edge. However, it has a minor influence on the shape of the $\text{Ce}\text{ }{L}_{2,3}$ XMCD spectra.