Core Electron Binding Energies Research Articles

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.

Processing and interpretation of core-electron XPS spectra of complex plasma-treated polyethylene-based surfaces using a theoretical peak model.

Interpretation of X-ray photoelectron spectroscopy (XPS) spectra of complex material surfaces, such as those obtained after surface plasma treatment of polymers, is confined by the available references. The limited understanding of the chemical surface composition may impact the ability to determine suitable coupling chemistries used for surface decoration or assess surface-related properties like biocompatibility. In this work, XPS is used to investigate the chemical composition of various ultra-high-molecular-weight polyethylene (UHMWPE) surfaces. UHMWPE doped with α-tocopherol or functionalised by active screen plasma nitriding (ASPN) was investigated as a model system. Subsequently, a more complex combined system obtained by ASPN treatment of α-tocopherol doped UHMWPE was investigated. Through ab initio orbital calculations and by employing Koopmans' theorem, the core-electron binding energies (CEBEs) were evaluated for a substantial number of possible chemical functionalities positioned on PE-based model structures. The calculated ΔCEBEs showed to be in reasonable agreement with experimental reference data. The calculated ΔCEBEs were used to develop a material-specific peak model suitable for the interpretation of merged high-resolution C 1s, N 1s and O 1s XPS spectra of PE-based materials. In contrast to conventional peak fitting, the presented approach allowed the distinction of functionality positioning (i.e. centred or end-chain) and evaluation of the long-range effects of the chemical functionalities on the PE carbon backbone. Altogether, a more detailed interpretation of the modified UHMWPE surfaces was achieved whilst reducing the need for manual input and personal bias introduced by the spectral analyst.

A localized view on molecular dissociation via electron-ion partial covariance

Inner-shell photoelectron spectroscopy provides an element-specific probe of molecular structure, as core-electron binding energies are sensitive to the chemical environment. Short-wavelength femtosecond light sources, such as Free-Electron Lasers (FELs), even enable time-resolved site-specific investigations of molecular photochemistry. Here, we study the ultraviolet photodissociation of the prototypical chiral molecule 1-iodo-2-methylbutane, probed by extreme-ultraviolet (XUV) pulses from the Free-electron LASer in Hamburg (FLASH) through the ultrafast evolution of the iodine 4d binding energy. Methodologically, we employ electron-ion partial covariance imaging as a technique to isolate otherwise elusive features in a two-dimensional photoelectron spectrum arising from different photofragmentation pathways. The experimental and theoretical results for the time-resolved electron spectra of the 4d3/2 and 4d5/2 atomic and molecular levels that are disentangled by this method provide a key step towards studying structural and chemical changes from a specific spectator site.

Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in [formula omitted] thin films

Ab initio calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti1−δOxN1−x). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by ∼0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti1−δOxN1−x thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti1−δOxN1−x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.

W 4f electron binding energies in amorphous W-B-C systems

In this paper, we critically evaluate the applicability of the procedure proposed in [Mirzaei et al., Surf. Coat. Technol. 358 (2019) 843–849] which is based on the fitting of the XPS spectrum of amorphous W-B-C material into three components with fixed peak positions to get the relative amount of W-W, W-B, and W-C bonds. We show that W-W bonds substantially influence positions of the peak components. We have verified this assumption by generating a set of models of amorphous W-B-C with different compositions (W:B:C ratio) and calculating the W 4f core electron binding energies employing ab initio methods. This enabled us to formulate the relationship between the W 4f electron binding energies (BE) and the local atomic environments of W atoms. Our analysis confirms the expected W4f chemical shifts in W-B-C caused by W-B and W-C bonds and reveals that W-W bonds shift the W 4f electronic states in the same direction as W-B bonds, which has substantial implications for the correct interpretation of the measured XPS spectra.

Δ‐SCF calculations of core electron binding energies in first‐row transition metal atoms

AbstractCore electron binding energies (CEBE) of nickel and copper atoms have been calculated using single‐configuration energy differences, the so‐called Δ‐self‐consistent field (Δ‐SCF) method. Basis set convergence has been examined for calculated L‐shell and M‐shell core electron binding energies, and a wide array of density functionals have been evaluated. Scalar relativistic corrections have been estimated using the popular Douglas‐Kroll‐Hess (DKH) approximation. While basis set convergence and functional dependence mirror the behavior reported in the literature for main group elements, the simplest Δ‐SCF calculations with pure Hartree‐Fock (HF) exchange and no correlation surprisingly outperform all density functionals in reproducing free‐atom CEBE values for Ni and Cu.

Predicting core electron binding energies in elements of the first transition series using the Δ-self-consistent-field method.

The Δ-Self-Consistent-Field (ΔSCF) method has been established as an accurate and computationally efficient approach for calculating absolute core electron binding energies for light elements up to chlorine, but relatively little is known about the performance of this method for heavier elements. In this work, we present ΔSCF calculations of transition metal (TM) 2p core electron binding energies for a series of 60 molecular compounds containing the first row transition metals Ti, V, Cr, Mn, Fe and Co. We find that the calculated TM 2p3/2 binding energies are less accurate than the results for the lighter elements with a mean absolute error (MAE) of 0.73 eV compared to experimental gas phase photoelectron spectroscopy results. However, our results suggest that the error depends mostly on the element and is rather insensitive to the chemical environment. By applying an element-specific correction to the binding energies the MAE is reduced to 0.20 eV, similar to the accuracy obtained for the lighter elements.

Analysis of AES of 2nd Period Element-containing Substances and Valence XPS of the Four Solid Ones by DFT Calculations Using the Model Molecules

Experimental Auger electron spectra (AES) of 2nd period elements and valence X-ray photoelectron spectra (VXPS) of four solid substances [LiF, graphite, GaN, SiO2] are analyzed by density-functional theory (DFT) calculations using the model molecules of the unit cell. For the calculations, we use Gaussian09 program at B3LYP/6-31G (d, p) level to estimate VXPS, core-electron binding energies, and (Li ~ F)-KVV AES of the solid substances. In the AES simulations, we evaluate theoretical kinetic energies of the AES with our modified calculation method. The modified kinetic energies correspond to the final-state holes at the ground state in DFT calculations. Simulated KVV AES of the (Li ~ O) atoms with the maximum kinetic energies of the atoms agree considerably well to the experimental AES results. Calculated VXPS of the four substances are also in good accordance with the experimental ones. The kinetic energy formula of AES at the ground state appears to work better, despite its great simplicity.

Simulating core electron binding energies of halogenated species adsorbed on ice surfaces and in solution via relativistic quantum embedding calculations.

In this work, we investigate the effects of the environment on the X-ray photoelectron spectra of hydrogen chloride and chloride ions adsorbed on ice surfaces, as well as of chloride ions in water droplets. In our approach, we combine a density functional theory (DFT) description of the ice surface with that of halogen species using the recently developed relativistic core-valence separation equation of motion coupled cluster (CVS-EOM-IP-CCSD) via the frozen density embedding formalism (FDE), to determine the K and L1,2,3 edges of chlorine. Our calculations, which incorporate temperature effects through snapshots from classical molecular dynamics simulations, are shown to reproduce experimental trends in the change of the core binding energies of Cl- upon moving from a liquid (water droplets) to an interfacial (ice quasi-liquid layer) environment. Our simulations yield water valence band binding energies in good agreement with experiment, which vary little between the droplets and the ice surface. For halide core binding energies there is an overall trend for overestimating experimental values, though good agreement between theory and experiment is found for Cl- in water droplets and on ice. For HCl on the other hand there are significant discrepancies between experimental and calculated core binding energies when we consider structural models that maintain the H-Cl bond more or less intact. An analysis of models that allow for pre-dissociated and dissociated structures suggests that experimentally observed chemical shifts in binding energies between Cl- and HCl would require that H+ (in the form of H3O+) and Cl- are separated by roughly 4-6 Å.

Efficient basis sets for core-excited states motivated by Slater's rules.

X-Ray photoemission spectroscopy is a commonly applied characterization technique that probes the local chemistry of atoms in molecules and materials via the photoexcitation of electrons from atomic core orbitals. These measurements can be interpreted by comparison with previous literature or through the calculation of core-electron binding energies (CEBEs) for model systems. However, physically and numerically accurate description of the core-excited electronic structures demands specializations beyond routine ground state setups. Inspired by Slater's rules, we focus on developing computationally efficient and physically motivated contractions to reproduce the core-excited atomic orbitals which led to improved numerical accuracy of calculated CEBEs. When applied to carbon 1s excitations in a wide range of molecules, these core-excited basis sets produce total energy differences (ΔSCF) using a hybrid exact-exchange density functional (B3LYP) that can reproduce core-excitation energies within experimental accuracy (∼0.1 eV). Due to missing relativistic effects, heavier elements (N, O) exhibit slightly larger systematic absolute errors, but still maintain a satisfactory 0.2 eV mean average error for relative CEBEs. We also connect the known variability in the core level binding energy with local atomic charge to demonstrate how the transferability of a given model should be measured against a diverse test set. We conclude by exploring one outlier, CO, and the outlook for extending this approach to other elements.

Electrodeposition of Indium from an Ionic Liquid Investigated by In Situ Electrochemical XPS

The electrochemical behavior and electrodeposition of indium in an electrolyte composed of 0.1 mol/L InCl3 in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI) on a gold electrode were investigated. The cyclic voltammogram revealed several reduction and oxidation peaks, indicating a complex electrochemical behavior. In the cathodic regime, with the formation of an In-Au alloy, the reduction of In(III) to In(I) and of In(I) to In(0) takes place. In situ electrochemical X-ray photoelectron spectroscopy (XPS) was employed to investigate the reduction process by monitoring the oxidation states of the components during the cathodic polarization of 0.1 mol/L InCl3/[Py1,4]TFSI on a gold working electrode under ultra-high vacuum (UHV) conditions. The core electron binding energies of the IL components (C 1s, O 1s, F 1s, N 1s, and S 2p) shift almost linearly to more negative values as a function of the applied cell voltage. At −2.0 V versus Pt-quasi reference, In(I) was identified as the intermediate species during the reduction process. In the anodic regime, a strong increase in the pressure in the XPS chamber was recorded at a cell voltage of more than −0.5 V versus Pt quasi reference, which indicated, in addition to the oxidation reactions of In species, that the oxidation of Cl− occurs. Ex situ XPS and XRD results revealed the formation of metallic In and of an In-Au alloy.

Core Electron Binding Energies in Solids from Periodic All-Electron Δ-Self-Consistent-Field Calculations.

Theoretical calculations of core electron binding energies are required for the interpretation of experimental X-ray photoelectron spectra, but achieving accurate results for solids has proven difficult. In this work, we demonstrate that accurate absolute core electron binding energies in both metallic and insulating solids can be obtained from periodic all-electron Δ-self-consistent-field (ΔSCF) calculations. In particular, we show that core electron binding energies referenced to the valence band maximum can be obtained as total energy differences between two (N - 1)-electron systems: one with a core hole and one with an electron removed from the highest occupied valence state. To achieve convergence with respect to the supercell size, the analogy between localized core holes and charged defects is exploited. Excellent agreement between calculated and experimental core electron binding energies is found for both metals and insulators, with a mean absolute error of 0.24 eV for the systems considered.

An improved Slater's transition state approximation.

We have extended Slater's transition state concept for the approximation of the difference in total energies of the initial and final states by three orbital energies of initial, final, and half-way Slater's transition states of the system. Numerical validation was performed with the ionization energies for H2O, CO, and pyrrole by calculation using Hartree-Fock (HF) and Kohn-Sham (KS) theories with the B3LYP and LCgau-core-BOP functionals. The present extended method reproduces full ΔSCF very accurately for all occupied orbitals obtained with HF and for valence orbitals obtained with KS. KS core orbitals have some errors due to the self-interaction errors, but the present method significantly improves the core electron binding energies. In its current form, the newly derived theory may not yet be practically useful, but it is simple and conceptually useful for gaining improved understanding of SCF-type orbital theories.

Density Functional Theory Calculations of Core-Electron Binding Energies at the K-Edge of Heavier Elements.

The capability to determine core-electron binding energies (CEBEs) is vital in the analysis of X-ray photoelectron spectroscopy, and the continued development of light sources has made inner shell spectroscopy of heavier elements increasingly accessible. Density functional theory is widely used to determine CEBEs of lighter elements (boron-fluorine). It is shown that good performance of exchange-correlation functionals for these elements does not necessarily translate to the calculation of CEBEs for the heavier elements from the next row of the periodic table, and in general, larger errors are observed. Two strategies are explored that improve the accuracy of the calculated CEBEs. The first is to apply element and functional dependent energy corrections, and the second is a reparametrization of a short-range corrected functional. This functional is able to reproduce experimental phosphorus and sulfur K-edge CEBEs with an average error of 0.15 eV demonstrating the importance of reducing the self-interaction error associated with the core electrons and represents progress toward a density functional theory calculation that performs equally well for ionization at the K-edge of all elements.

Single crystal growth and the electronic structure of Rb2Na(NO3)3: Experiment and theory

Rb2Na(NO3)3 crystals demonstrate nonlinear optical properties and can be used as a converter of laser radiation in the shortwave region. The crystals were grown in the present work by the Bridgman–Stockbarger method in a ratio of 75 ​wt%(RbNO3) and 25 ​wt%(NaNO3). After the growth, a transparent centimeter size single crystal (6 ​cm long) was obtained for the first time that is very important for its practical application. The unit cell volume of double Rb2Na(NO3)3 nitrate is intermediate between the cell volumes of simple rubidium and sodium nitrates, RbNO3 and NaNO3. Electronic structure of Rb2Na(NO3)3 was studied in the present work from both experimental and theoretical viewpoints. In particular, employing X-ray photoelectron spectroscopy, we have measured binding energies of core electrons and energy distribution of the electronic states within the valence band region of the Rb2Na(NO3)3 crystal and established rather big binding energies for N 1s and O 1s core-level electrons. The bombardment of middle-energy Ar+ ions induces transformation of some nitrogen atoms of the analyzing topmost layers of the Rb2Na(NO3)3 crystal surface from the NO3– group to the NO2– group. To explore in detail the filling of the valence band of Rb2Na(NO3)3 by electronic states associated with constituting atoms, we use first-principles calculations within a density functional theory (DFT) framework. The DFT calculations reveal that O 2p states are the principal contributors to the valence band bringing the main input in its upper portion. The theoretical finding is supported experimentally by fitting the X-ray photoelectron valence band spectrum and the X-ray emission O Kα band on the total energy scale. The conduction band bottom of Rb2Na(NO3)3 is composed by unoccupied O 2p and N 2p states in almost equal proportion.