Core Electron Binding Energies Research Articles

Peak Broadening in Photoelectron Spectroscopy of Amorphous Polymers: The Leading Role of the Electrostatic Landscape.

The broadening in photoelectron spectra of polymers can be attributed to several factors, such as light source spread, spectrometer resolution, the finite lifetime of the hole state, and solid-state effects. Here, for the first time, we set up a computational protocol to assess the peak broadening induced for both core and valence levels by solid-state effects in four amorphous polymers by using a combination of density functional theory, many-body perturbation theory, and classical polarizable embedding. We show that intrinsic local inhomogeneities in the electrostatic environment induce a Gaussian broadening of 0.2-0.7 eV in the binding energies of both core and semivalence electrons, corresponding to a full width at half-maximum (FWHM) of 0.5-1.7 eV for the investigated systems. The induced broadening is larger in acrylate-based than in styrene-based polymers, revealing the crucial role of polar groups in controlling the roughness of the electrostatic landscape in the solid matrix.

Characterization of peroxide on coal surface: A combined △SCF and XPS study

X‐ray photoelectron spectroscopy (XPS) is widely used to determine the composition of elements and oxygen‐containing functional groups. Accurate determination of core‐electron binding energies (CEBEs) is essential for identifying the types and contents of oxygen‐functional groups present on the surface of coal. The △SCF method has been carried out on CEBEs for an extensive series of oxygen‐containing organic systems encompassing the most common functionalities. The polymers for which experimental values are available for comparison demonstrate the adequacy of the treatment for describing CEBEs for the O1s core level. Therefore, a sound basis is provided for the discussion of CEBEs (O1s) for functionalities for which experimental values are not accurate or available, such as carbonyl, carboxyl, and peroxide. The results are applied to the XPS study of oxidized coal. The change law of peroxide and oxygen‐containing functional groups was discussed. The calculation method and results in this paper can provide a guide for readers to use XPS to study the oxidation of coal surfaces.

Tuning Pd-to-Ag Ratio to Enhance the Synergistic Activity of Fly Ash-Supported PdxAgy Bimetallic Nanoparticles.

Fly ash (FA)-supported bimetallic nanoparticles (PdxAgy/FA) with varying Pd:Ag ratios were prepared by coprecipitation of Pd and Ag involving in situ reduction of Pd(II) and Ag(I) salts in aqueous medium. All the supported nanoparticles were thoroughly characterized with the aid of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM)), and elemental analyses, which include inductively coupled plasma-optical emission spectroscopy (ICP-OES) and energy-dispersive X-ray spectroscopy (EDS). A gradual broadening and shifting of PXRD peaks, ascribable to Ag, to higher angles with an increase in the Pd:Ag ratio affirms the alloying of interface between Pd and Ag nanoparticles. The coexistence of Pd and Ag was further confirmed by EDS elemental mapping as well as by the presence of bimetallic lattices on the FA surface, as evident from the high-resolution TEM analysis. The dependency of crystallite size and average size of bimetallic nanoparticles on Ag loading (mol %) was elucidated with the help of a combination of PXRD and TEM studies. Based on XPS analysis, the charge transfer phenomenon between contacting Pd-Ag sites could be evident from the shifting of 3d core electron binding energy for both Pd and Ag compared with monometallic Pd and Ag nanoparticles. Following a pseudo-first-order reaction kinetics, all the nanocatalysts were able to efficiently reduce 4-nitrophenol into 4-aminophenol in aqueous NaBH4. The superior catalytic performance of the bimetallic nanocatalysts (PdxAgy/FA) over their monometallic (Pd100/FA and Ag100/FA) analogues has been demonstrated. Moreover, the tunable synergistic effect of the bimetallic systems has been explored in detail by varying the Pd:Ag mol ratio in a systematic manner which in turn allowed us to achieve an optimum reaction rate (k = 1.050 min-1) for the nitrophenol reduction using a Pd25Ag75/FA system. Most importantly, all the bimetallic nanocatalysts explored here exhibited excellent normalized rate constants (K ≈ 6000-15,000 min-1 mmol-1) compared with other supported bimetallic Pd-Ag nanocatalysts reported in the literature.

Δ-based composite models for calculating x-ray absorption and emission energies

A practical ab initio composite method for modeling x-ray absorption and non-resonant x-ray emission is presented. Vertical K-edge excitation and emission energies are obtained from core-electron binding energies calculated with spin-projected ΔHF/ΔMP and outer-core ionization potentials/electron affinities calculated with electron propagator theory. An assessment of the combined methodologies against experiment is performed for a set of small molecules containing second-row elements.

Specific versus Nonspecific Solvent Interactions of a Biomolecule in Water.

Solvent interactions, particularly hydration, are vital in chemical and biochemical systems. Model systems reveal microscopic details of such interactions. We uncover a specific hydrogen-bonding motif of the biomolecular building block indole (C8H7N), tryptophan's chromophore, in water: a strong localized N-H···OH2 hydrogen bond, alongside unstructured solvent interactions. This insight is revealed from a combined experimental and theoretical analysis of the electronic structure of indole in aqueous solution. We recorded the complete X-ray photoemission and Auger spectrum of aqueous-phase indole, quantitatively explaining all peaks through ab initio modeling. The efficient and accurate technique for modeling valence and core photoemission spectra involves the maximum-overlap method and the nonequilibrium polarizable-continuum model. A two-hole electron-population analysis quantitatively describes the Auger spectra. Core-electron binding energies for nitrogen and carbon highlight the specific interaction with a hydrogen-bonded water molecule at the N-H group and otherwise nonspecific solvent interactions.

Combining the Δ-Self-Consistent-Field and GW Methods for Predicting Core Electron Binding Energies in Periodic Solids.

For the computational prediction of core electron binding energies in solids, two distinct kinds of modeling strategies have been pursued: the Δ-Self-Consistent-Field method based on density functional theory (DFT), and the GW method. In this study, we examine the formal relationship between these two approaches and establish a link between them. The link arises from the equivalence, in DFT, between the total energy difference result for the first ionization energy, and the eigenvalue of the highest occupied state, in the limit of infinite supercell size. This link allows us to introduce a new formalism, which highlights how in DFT─even if the total energy difference method is used to calculate core electron binding energies─the accuracy of the results still implicitly depends on the accuracy of the eigenvalue at the valence band maximum in insulators, or at the Fermi level in metals. We examine whether incorporating a quasiparticle correction for this eigenvalue from GW theory improves the accuracy of the calculated core electron binding energies, and find that the inclusion of vertex corrections is required for achieving quantitative agreement with experiment.

Intramolecular O···H Hydrogen Bonding of Salicylic Acid: Further Insights from O 1s XPS and 1H NMR Spectra Using DFT Calculations.

Intramolecular hydrogen bonding (HB) is a complex phenomenon that extends beyond a simple valence event, affecting the core electrons of a molecule. Salicylic acid (SA) and its conformers provide an excellent model compound for studying intramolecular HB as the proton donor (H) and acceptor (O) can be toggled by rotating the C-O and C-C bonds to form up to seven potential conformers through various HB. In this study, we computationally investigated intramolecular interactions in SA conformers with and without such HB, by examining their calculated O 1s core electron-binding energy (CEBE) and 1H NMR chemical shifts validated using recent measurements. The quantum mechanically stable SA conformers are fully defined by three rotatable bonds in the compound, which are abstracted as SA(η1η2η3) digital structures, where ηi = 0 if the ηi angles match the most stable SA conformer (000) and ηi = 1 otherwise. Our findings suggest that the stability is dominated by the appearance of the intergroup intramolecular HB of Hp···O (where O is in the carboxylic acid functional group and Hp is the phenolic proton in -OHp), and η3 serves as a switch of such HB. As a result, the (η1η20) SA conformers containing such Hp···O HB are more stable than other SA conformers (η1η21) without such the Hp···O HB. The present density functional theory calculations reveal that this Hp···O HB results in splitting of the O 1s CEBEs of two hydroxyl groups (-OH) by up to 1 eV and deshielding the Hp proton 1H NMR (δHp) up to 11.68 ppm for the (η1η20) conformers. Without such Hp···O HB, the O 1s XPS binding energies of two -OH groups will be closely located in the same band, and the 1H NMR chemical shift of the Hp atom will be as small as an 4.09 ppm SA conformer [SA-G(101)]. The present study indicates that the O 1s CEBE splitting between two -OH groups serves as an indicator of the presence of the Hp···O HB in SA conformers, which is also supported by the 1H NMR results.

Spatiotemporal Changes in Atomic and Molecular Architecture of Mineralized Bone under Pathogenic Conditions

Mechanisms responsible for spatiotemporal changes in the atomic-molecular architecture of the human femur in intact and osteoarthritis-affected areas were studied using high-resolution X-ray diffraction and spectroscopic techniques. Comparison of the experimental data demonstrates strong deviations of core electron-binding energies, lattice constants of hydroxyapatite crystal cells, linear sizes of crystallites, and degrees of crystallinity for both intact and osteoarthritic areas. The quantitative values of these characteristics and their standard deviations in each area are measured and presented. A systematic analysis of the site-dependent deviations was carried out within the framework of the 3D superlattice model. It is argued that the main mechanism responsible for the deviations arises primarily as a result of carbonization and catalytic reactions at the mineral-cartilage interface. The impact of the mechanism is enhanced in the vicinities of the area of sclerosed bone, but not inside the area where mechanical loads are maximum. Restoration of the atomic-molecular architecture of mineralized bone in the sclerosis area is revealed. Statistical aspects of the spatiotemporal changes in mineralized bone under pathogenic conditions are discussed.

Breakdown of the correlation between oxidation states and core electron binding energies at the sub-nanoscale

X-ray photoelectron spectroscopy is a powerful analytical tool to fingerprint the atomic oxidation state. However, the well-established trend that is found for bulk, ultra-thin films and surface oxides, which is usually characterized by an incremental binding energy shift upon increasing oxidation state, may not apply at the sub-nanoscale where reduced dimensionality and quantum size effects play a relevant role. Here we investigate through a combined experimental and theoretical approach the oxidation of size-selected Ag7 and Ag11 clusters supported on graphene, revealing an anomalous Ag 3d5/2 core level shift trend upon increasing O coverage. We show that the negative core level shift trend typical of Ag reverts back in the case of the highest Ag(III) oxidation state, an effect that is due to the peculiar electronic structure of Ag nano-oxides. Our results highlight the great care needed to extend at the zero-dimensional scale the knowledge acquired for the spectral interpretation of 3D and 2D materials.

XPS Core-Level Chemical Shift by Ab Initio Many-Body Theory.

X-ray photoemission spectroscopy (XPS) provides direct information on atomic composition and stoichiometry by measuring core-electron binding energies. Moreover, from the shift of the binding energy, the so-called chemical shift, the precise chemical type of bonds can be inferred, which brings additional information on the local structure. In this work, we present a theoretical study of the chemical shift first by comparing different theories, from Hartree-Fock and density functional theory to many-body perturbation theory approaches like the GW approximation and its static version (COHSEX). The accuracy of each theory is assessed with respect to a carbon 1s chemical shift experimental benchmark measured on a set of gas-phase molecules. More importantly, by decomposing the chemical shift into different contributions according to terms in the total Hamiltonian, classical electrostatics is identified as the major contributor to the chemical shift, one order of magnitude larger than the correlation.

Projected Hybrid Density Functionals: Method and Application to Core Electron Ionization.

This work introduces a new class of hybrid density functional theory (DFT) approximations, which incorporate different fractions of nonlocal exact exchange in predefined states such as core atomic orbitals (AOs). These projected hybrid density functionals are related to range-separated hybrid functionals, which incorporate different fractions of nonlocal exchange at different electron-electron separations. This work derives projected hybrids using the Adiabatic Projection formalism. One projects the electron-electron interaction operator onto the chosen predefined states, introduces the projected operator into the noninteracting Kohn-Sham reference system, and employs a formally exact density functional to model the remaining electron-electron interactions. Projected hybrids, like range-separated hybrids, approximate the partially interacting reference system's ground-state wave function as a single Slater determinant. Projected hybrids are readily implemented into existing density functional codes, requiring only a projection of the one-particle density matrices and exchange operators entering existing routines. This work also presents an application to core electron ionization. Projecting onto core atomic orbitals allows us to introduce additional nonlocal exchange into atomic core regions. This reduces the impact of self-interaction error on computed core electron properties. Benchmark studies are reported for PBE0c70, a core-projected variant of the Perdew-Burke-Ernzerhof global hybrid PBE0, in which the fraction of nonlocal exchange is increased from 25% to 70% in atomic core regions. PBE0c70-predicted core orbital energies accurately recover nonrelativistic core-electron binding energies of second-period elements Li-Ne and third-period elements Na-Ar, without degrading the good performance of PBE0 for atomization energies and valence ionization potentials.

Electronic structure study of H3BXH3 (X═B, N and P) as hydrogen storage materials using calculated NMR and XPS spectra

Boron-based materials have been used for hydrogen storage applications owing to their high volumetric and gravimetric hydrogen density. The present study quantum mechanically investigates the electronic structures of three compounds: diborane (DB, B2H6), ammonia borane (AB, H3BNH3) and phosphine borane (PB, H3BPH3). The exploration is facilitated using calculated nuclear magnetic resonance (NMR) chemical shifts, together with outer valence ionisation potentials (IP) and core electron binding energy (CEBE). The findings show a distinct electronic structure for diborane, differing notably from AB and PB, which exhibit certain similarities. Noteworthy dissimilarities are observed in the chemical environments of the bridge hydrogens and terminal hydrogens in diborane, resulting in a substantial chemical shift difference of up to 5.31 ppm. Conversely, in AB and PB, two distinct sets of hydrogens emerge: protic hydrogens (Hp–N and Hp–P) and hydridic hydrogens (Hh–B). This leads to chemical shifts as small as 0.42 ppm in AB and as significant as 3.0 ppm in PB. The absolute isotropic NMR shielding constant (σB) of 11B in DB is 85.40 ppm, in contrast to 126.21 ppm in AB and 151.46 ppm in PB. This discrepancy indicates that boron in PB has the most robust chemical environment among the boranes. This assertion finds support in the calculated CEBE for B 1s of 196.53, 194.01 and 193.93 eV for DB, AB and PB respectively. It is clear that boron in PB is the most reactive atom. Ultimately, understanding the chemical environment of the boranes is pivotal in the context of dehydrogenation processes for boron-based hydrogen storage materials.

Observation of site-selective chemical bond changes via ultrafast chemical shifts

The concomitant motion of electrons and nuclei on the femtosecond time scale marks the fate of chemical and biological processes. Here we demonstrate the ability to initiate and track the ultrafast electron rearrangement and chemical bond breaking site-specifically in real time for the carbon monoxide diatomic molecule. We employ a local resonant x-ray pump at the oxygen atom and probe the chemical shifts of the carbon core-electron binding energy. We observe charge redistribution accompanying core-excitation followed by Auger decay, eventually leading to dissociation and hole trapping at one site of the molecule. The presented technique is general in nature with sensitivity to chemical environment changes including transient electronic excited state dynamics. This work provides a route to investigate energy and charge transport processes in more complex systems by tracking selective chemical bond changes on their natural timescale.

Resonant double-core excitations with ultrafast, intense X-ray pulses

Intense few-to-sub-femtosecond soft X-ray pulses can produce neutral, two-site excited double-core-hole states by promoting two core electrons to the same unoccupied molecular orbital. We theoretically investigate double nitrogen K-edge excitations of nitrous oxide (N 2 O) with multiconfigurational electronic structure calculations. We show that the second core-excitation energy is reduced with respect to its ground state value. A site-selective double core-excitation mechanism using intense few-femtosecond x-rays is investigated using time-dependent Schrödinger equation (TDSE) simulations. The subsequent two-step Auger–Meitner and two-electrons-one-electron decay spectra of the double core-excited states are analysed using a Mulliken population analysis of the multiconfirational wavefunctions. The change in the electron emission lineshape between the absorption of 1 or 2 photons in the resonant core-excitation is predicted by combining this approach with the TDSE simulations. We examine the possibility of resolving the double core-excited states with X-ray pump-probe techniques by calculating the chemical shifts of the core-electron binding energy of the core-excited states and decay products.

Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study.

Ammonia borane (H3BNH3) is a promising material for hydrogen storage and release. Dehydrogenation of ammonia borane produces small boron–nitrogen hydrides such as aminoborane (H2BNH2) and iminoborane (HBNH). The present study investigates ammonia borane and its two dehydrogenated products for the first time using calculated photoemission spectra of the valence and core electrons. It is found that a significant decrease in the dipole moment was observed associated with the dehydration from 5.397 D in H3BNH3, to 1.942 D in H2BNH2, and to 0.083 D in HBNH. Such reduction in the dipole moment impacts properties such as hydrogen bonding, dihydrogen bonding, and their spectra. Dehydrogenation of H3BNH3 impacts both the valence and core electronic structure of the boron–nitrogen hydrides. The calculated valence vertical ionization energy (VIE) spectra of the boron–nitrogen hydrides show that valence orbitals dominated by 2p-electrons of B and N atoms exhibit large changes, whereas orbitals dominated by s-electrons, such as (3a14a15a1/3σ4σ5σ) remain less affected. The first ionization energy slightly increases from 10.57 eV for H3BNH3 to 11.29 eV for both unsaturated H2BNH2 and HBNH. In core space, the oxidative dehydrogenation of H3BNH3 affects the core electron binding energy (CEBE) of borane and nitrogen oppositely. The B1s binding energies increase from 194.01 eV in H3BNH3 to 196.93 eV in HBNH, up by 2.92 eV, whereas the N1s binding energies decrease from 408.20 eV in H3BNH3 to 404.88 eV in HBNH, dropped by 3.32 eV.

Modeling X-ray Photoelectron Spectroscopy of Macromolecules Using GW.

We propose a simple additive approach to simulate X-ray photoelectron spectra (XPS) of macromolecules based on the GW method. Single-shot GW (G0W0) is a promising technique to compute accurate core-electron binding energies (BEs). However, its application to large molecules is still unfeasible. To circumvent the computational cost of G0W0, we break the macromolecule into tractable building blocks, such as isolated monomers, and sum up the theoretical spectra of each component, weighted by their molar ratio. In this work, we provide a first proof of concept by applying the method to four test polymers and one copolymer and show that it leads to an excellent agreement with experiments. The method could be used to retrieve the composition of unknown materials and study chemical reactions, by comparing the simulated spectra with experimental ones.

Real-Space Pseudopotential Method for the Calculation of 1s Core-Level Binding Energies.

We systematically studied a real-space pesudopotential method for the calculation of 1s core–electron binding energies of second-row elements B, C, N, and O within the framework of Kohn–Sham density functional theory (KS-DFT). With Dirichlet boundary conditions, pseudopotential calculations can provide accurate core–electron binding energies for molecular systems, when compared with the results from all-electron calculations and experiments. Furthermore, we report that with one simple additional nonself-consistent calculation as a refinement step using a hybrid exchange-correlation functional, we can generally improve the accuracy of binding energy shifts, promising a strategy for improving accuracy at a much lower computational cost. The specializations in the present approach, combined with our efficient real-space KS-DFT implementation, provide key advantages for calculating accurate core–electron binding energies of large-scale systems.

Electron removal energies in noble-gas atoms up to 100 keV: Ab initio GW versus x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) measures electron removal energies, providing direct access to core and valence electron binding energies, hence probing the electronic structure. In this Letter, we benchmark the ab initio many-body $GW$ approximation on the complete electron binding energies of noble-gas atoms (He-Rn), which span 100 keV. Our results demonstrate that $GW$ achieves an accuracy within 1.2% in XPS binding energies, by systematically restoring the underestimation from density-functional theory (error of 14%) or the overestimation from Hartree-Fock (error of 4.7%). Such results also imply the correlations of $d$ electrons are very well described by $GW$.

Accurate Computational Prediction of Core-Electron Binding Energies in Carbon-Based Materials: A Machine-Learning Model Combining Density-Functional Theory and GW.

We present a quantitatively accurate machine-learning (ML) model for the computational prediction of core–electron binding energies, from which X-ray photoelectron spectroscopy (XPS) spectra can be readily obtained. Our model combines density functional theory (DFT) with GW and uses kernel ridge regression for the ML predictions. We apply the new approach to disordered materials and small molecules containing carbon, hydrogen, and oxygen and obtain qualitative and quantitative agreement with experiment, resolving spectral features within 0.1 eV of reference experimental spectra. The method only requires the user to provide a structural model for the material under study to obtain an XPS prediction within seconds. Our new tool is freely available online through the XPS Prediction Server.

Wavefunction-based quantum-chemical ab initio calculations for core electron binding energies of small open shell molecules

Core electron binding energies (CEBEs), i.e. ionization energies of 1s core orbitals, are calculated by means of wavefunction-based quantum-chemical ab initio methods for a series of small open-shell molecules containing first-row atoms. The calculations are performed in three steps: (a) Koopmans’ theorem, where the orbitals of the electronic ground state are used unchanged also for the ions, (b) Hartree–Fock or self consistent field (SCF) approximation in which the orbitals are allowed to relax after 1s ionization (ΔSCF), (c) dynamic correlation effects on top of SCF. For open-shell molecules 1s ionization leads to ions in several spin states, mostly to a pair of a triplet and a singlet state. In several cases one or both of these ionic states are only poorly described by a single-reference SCF wavefunction, therefore a multi-reference complete active space self consistent field (CAS-SCF) wavefunction is used instead. The correlation effects are evaluated by means of our multi-reference coupled electron pair approximation program. The accuracy of the calculated CEBEs is in the order of 0.1–0.4 eV. This is in agreement with experimental results for NO and O2. But there exist only very few gas phase data for CEBEs of open-shell molecules.