Core Valence Research Articles

Strength analysis of noncovalent interactions between lignite and direct liquefaction solvents: A joint study of DFT calculations and swelling ratio determination

Noncovalent interactions have achieved extensive attentions in thermal conversion of low rank coal, but have been little studied in direct coal liquefaction (DCL). In order to investigate strength of noncovalent interactions (hydrogen bonds and π–π stacking) between lignite and DCL solvents, experimental methods and DFT (density function theory) calculations were utilized. Aromatic components in industrial recycled solvents have been selected, including tetralin, 1,2–dihydronaphthalene (DHN), indane, 1–methylnaphthalene, naphthalene, indene and benzene. On one hand, total strength of noncovalent interactions between those aromatic components and lignite were integrally evaluated by swelling ratio determination. On the other hand, Spatial distribution and strength of noncovalent interactions were investigated by RDG (reduced density gradient) analysis, CVB (core–valence bifurcation) index and LOLIPOP (localized orbital locator integrated pi over plane) index, respectively. Overall, addition of DHN, indane and benzene resulted in significant swelling of lignite. Swelling of lignite in benzene and others (DHN and indane) was enhanced by stronger OH–π hydrogen bonds and π–π stacking, respectively. By conjoint study of swelling ratio and DFT calculations, both hydrogen bonds and π–π stacking play crucial roles in swelling of lignite. Adjustment of noncovalent interactions should be taken into account during design of DCL solvents, based on correlations between noncovalent interactions and structural characteristics of aromatic components.

Unitary coupled-cluster approach for the calculation of core-excited states and x-ray absorption spectra.

In this work, we present the core-valence separation (CVS) approximation applied to unitary coupled-cluster (UCC) theory for the calculation of core-excited states and the simulation of x-ray absorption spectroscopy (XAS). Excitation energies and oscillator strengths of small- to medium-sized organic molecules have been computed using the second-order and extended second-order UCC schemes (CVS-UCC2 and CVS-UCC2-x) as well as the third-order scheme (CVS-UCC3). All results are compared to the corresponding algebraic-diagrammatic construction methods and experimental data. The agreement between CVS-UCC and experimental data demonstrates its potential as a new approach for the calculation of XAS.

Radiative lifetimes of A 2Π, B 2Σ+, a 4Σ+, and b 4Δ states of CP radical

The potential energy curves of X 2Σ+, A 2Π, B 2Σ+, a 4Σ+, and b 4Δ states and their Ω states of CP were calculated using the icMRCI approach. The transition dipole moments between these states were computed. To improve the accuracy of the potential energy curves, core–valence correlation and scalar relativistic corrections were considered. The radiative lifetimes were approximately 10 μs for the A 2Π3/2 and A 2Π1/2 states and 100 ns for the B2Σ+1/2 state; those were approximately 10, 10 – 100, 10, 0.1 – 1, and 10 ms for the a 4Σ+1/2, a 4Σ+3/2, b 4Δ1/2, b 4Δ3/2, and b 4Δ5/2 states, respectively. The spontaneous emissions from the B 2Σ+1/2 – X 2Σ+1/2 system were strong, followed by the A 2Π3/2 – X 2Σ+1/2, A 2Π1/2 – X 2Σ+1/2, B 2Σ+1/2 – A 2Π3/2, and B 2Σ+1/2 – A 2Π1/2 transitions. The strong emissions from the A 2Π3/2 – X 2Σ+1/2 and A 2Π1/2 – X 2Σ+1/2 systems were in the infrared region; those from the B 2Σ+1/2 – X 2Σ+1/2 system were in the visible and ultraviolet regions; and those from the B 2Σ+1/2 – A 2Π3/2, and B 2Σ+1/2 – A 2Π1/2 systems were of visible light. A simple comparison shows that the effect of the spin–orbit coupling on the radiative lifetimes of the A 2Π and B 2Σ+ states was significant. The radiative–lifetime distribution of each level υ versus the rotational quantum number J was evaluated for υ ≤ 15 and J ≤ 70.5.

Feshbach-Fano approach for calculation of Auger decay rates using equation-of-motion coupled-cluster wave functions. II. Numerical examples and benchmarks.

X-ray photon absorption leads to the creation of highly excited species, which often decay through the Auger process. The theoretical treatment of Auger decay is challenging because of the resonance nature of the initial core-excited or core-ionized states and the continuous nature of the ejected electron. In Paper I [W. Skomorowski and A. I. Krylov, J. Chem. Phys. 154, 084124 (2021)], we have introduced a theoretical framework for computing Auger rates based on the Feshbach-Fano approach and the equation-of-motion coupled-cluster ansätze augmented with core-valence separation. The outgoing Auger electron is described with a continuum orbital. We considered two approximate descriptions-a plane wave and a Coulomb wave with an effective charge. Here, we use the developed methodology to calculate Auger transition rates in core-ionized and core-excited benchmark systems (Ne, H2O, CH4, and CO2). Comparison with the available experimental spectra shows that the proposed computational scheme provides reliable ab initio predictions of the Auger spectra. The reliability, cost efficiency, and robust computational setup of this methodology offer advantages in applications to a large variety of systems.

Feshbach–Fano approach for calculation of Auger decay rates using equation-of-motion coupled-cluster wave functions. I. Theory and implementation

X-ray absorption creates electron vacancies in the core shell. These highly excited states often relax by Auger decay-an autoionization process in which one valence electron fills the core hole and another valence electron is ejected into the ionization continuum. Despite the important role of Auger processes in many experimental settings, their first-principles modeling is challenging, even for small systems. The difficulty stems from the need to describe many-electron continuum (unbound) states, which cannot be tackled with standard quantum-chemistry methods. We present a novel approach to calculate Auger decay rates by combining Feshbach-Fano resonance theory with the equation-of-motion coupled-cluster single double (EOM-CCSD) framework. We use the core-valence separation scheme to define projectors into the bound (square-integrable) and unbound (continuum) subspaces of the full function space. The continuum many-body decay states are represented by products of an appropriate EOM-CCSD state and a free-electron state, described by a continuum orbital. The Auger rates are expressed in terms of reduced quantities, two-body Dyson amplitudes (objects analogous to the two-particle transition density matrix), contracted with two-electron bound-continuum integrals. Here, we consider two approximate treatments of the free electron: a plane wave and a Coulomb wave with an effective charge, which allow us to evaluate all requisite integrals analytically; however, the theory can be extended to incorporate a more sophisticated description of the continuum orbital.

Transition properties of X1Σ+, A1Σ−, B1Δ, C1Π, a3Σ+, b3Δ, c3Π, and d3Σ− states of PO+

The potential energy curves of the X1Σ+, A1Σ–, B1Δ, C1Π, a3Σ+, b3Δ, c3Π, and d3Σ– states of PO+ and the transition dipole moments between these states were calculated using the CASSCF method, and then the icMRCI approach. To accurately compute the transition properties, core–valence correlation and scalar relativistic corrections were taken into account. The spin–orbit coupling calculations were used to determine the transition properties of the A1Σ–, B1Δ, a3Σ+, and b3Δ states, from which no spontaneous emissions were generated owing to the limitations from the dipole–allowed transition selection rule. The radiative lifetime was approximately 10 – 100 ns for the C1Π state and 10 μs for the c3Π state. Among these dipole–allowed transitions, the emissions from the C1Π – X1Σ+ system was the strongest and was followed by the C1Π – A1Σ– and c3Π – a3Σ+ transitions. The radiative lifetime was approximately 10 – 100, 10 – 100, and 100 μs for the A1Σ–0–, d3Σ–0+, and d3Σ–1 states, respectively. Those were of the order of 0.1 – 10, 10, 10 – 100, 1 – 100, and 0.1 – 1 ms for the B1Δ2, a3Σ+1, a3Σ+0–, b3Δ1, and b3Δ2 states, respectively. Among the spontaneous emissions generated from these Ω states, those from the A1Σ–1 – a3Σ+0–, A1Σ–1 – a3Σ+1, B1Δ2 – b3Δ3, b3Δ2 – a3Σ+1, d3Σ–0+ – a3Σ+0–, and d3Σ–1 – a3Σ+1 transitions were relatively strong. The transition properties reported herein are expected to provide useful guidelines for the detection in future experiments.

Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer.

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS2 monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS2 core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (∼0.5 eV) at the MoS2 ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.

Электронная структура термически окисленного вольфрама

Using the method of photoelectron spectroscopy, an in situ study in ultrahigh vacuum of the electronic structure of a clean surface of tungsten oxidized at an oxygen pressure of 1 Torr and temperature of 1000 K was carried out. The spectra of photoemission from the valence band and core levels O 1s, O 2s, W 4f under synchrotron excitation in the photon energy range 80  600 eV are studied. It was found that a semiconductor film of tungsten oxide is formed, which contains various tungsten oxides with an oxidation state of 6+ to 4+. On the surface, mainly tungsten oxides with an oxidation state of 6+ are formed, the proportion of which gradually decreases with distance from the surface with an increase in tungsten oxides with an oxidation state of 4+.

Energy Levels and Radiative Rates in Ga XVII

Сomplete and consistent atomic data, including energy levels, wavelengths, lifetimes, and E1, E2, M1, and M2 transition rates, are reported for the low-lying 41 levels of Ga XVII, belonging to the n = 3 states (1s22s22p6)3s23p3, 3s3p4, and 3s23p23d. High-accuracy calculations act as benchmarks for accurate treatments of relativity, electronic correlation, and quantum electrodynamic (QED) effects in multi-valence-electron systems. The calculated energy levels are in excellent agreement with the experimental results and the experimentally compiled energy values of the National Institute for Standards and Technology wherever available. The calculated values, including core–valence correction, are found to be in good agreement with other theoretical and experimental values.

Energy Levels, Wavelengths, Probabilities, and Oscillator Strengths of Transition for Ge-Like Pd, Ag, and Cd Ions

High-accuracy calculations of energy levels, wavelengths, probabilities, and oscillator strengths of transition of resonance lines for Ge-like Pd, Ag, and Cd ions have been performed. For the accurate treatment of relativity, the contributions of Breit interactions and quantum electrodynamics correction were considered. The calculated values of energy levels and wavelengths, including core–valence corrections, are found to be in excellent agreement with other theoretical and experimental values for Ge-like Pd, Ag, and Cd ions. The number of energy levels and wavelengths we considered is larger than that of any other theoretical calculations. The transition probabilities are also given where no other theoretical results and experimental values are available.

Band Structure Extraction at Hybrid Narrow-Gap Semiconductor-Metal Interfaces.

The design of epitaxial semiconductor–superconductor and semiconductor–metal quantum devices requires a detailed understanding of the interfacial electronic band structure. However, the band alignment of buried interfaces is difficult to predict theoretically and to measure experimentally. This work presents a procedure that allows to reliably determine critical parameters for engineering quantum devices; band offset, band bending profile, and number of occupied quantum well subbands of interfacial accumulation layers at semiconductor‐metal interfaces. Soft X‐ray angle‐resolved photoemission is used to directly measure the quantum well states as well as valence bands and core levels for the InAs(100)/Al interface, an important platform for Majorana‐zero‐mode based topological qubits, and demonstrate that the fabrication process strongly influences the band offset, which in turn controls the topological phase diagrams. Since the method is transferable to other narrow gap semiconductors, it can be used more generally for engineering semiconductor–metal and semiconductor–superconductor interfaces in gate‐tunable superconducting devices.

Multiconfiguration Dirac–Hartree–Fock radiative parameters for emission lines in Ce ii–ivions and cerium opacity calculations for kilonovae

ABSTRACTLarge-scale calculations of atomic structures and radiative properties have been carried out for singly, doubly, and trebly ionized cerium. For this purpose, the purely relativistic multiconfiguration Dirac–Hartree–Fock (MCDHF) method was used, taking into account the effects of valence–valence and core–valence electronic correlations in detail. The results obtained were then used to calculate the expansion opacities characterizing the kilonovae observed as a result of neutron star mergers. Comparisons with previously published experimental and theoretical studies have shown that the results presented in this work are the most complete currently available, in terms of quantity and quality, concerning the atomic data and monochromatic opacities for Ce ii, Ce iii, and Ce iv ions.

Canonical and DLPNO-Based Composite Wavefunction Methods Parametrized against Large and Chemically Diverse Training Sets. 2: Correlation-Consistent Basis Sets, Core-Valence Correlation, and F12 Alternatives.

A hierarchy of wavefunction composite methods (cWFT), based on G4-type cWFT methods available for elements H through Rn, was recently reported by the present authors [J. Chem. Theor. Comput.2020, 16, 4238]. We extend this hierarchy by considering the inner-shell correlation energy in the second-order Møller–Plesset correction and replacing the Weigend–Ahlrichs def2-mZVPP(D) basis sets used with complete basis set extrapolation from augmented correlation-consistent core–valence triple-ζ, aug-cc-pwCVTZ(-PP), and quadruple-ζ, aug-cc-pwCVQZ(-PP), basis sets, thus creating cc-G4-type methods. For the large and chemically diverse GMTKN55 benchmark suite, they represent a substantial further improvement and bring WTMAD2 (weighted mean absolute deviation) down below 1 kcal/mol. Intriguingly, the lion’s share of the improvement comes from better capture of valence correlation; the inclusion of core–valence correlation is almost an order of magnitude less important. These robust correlation-consistent cWFT methods approach the CCSD(T) complete basis limit with just one or a few fitted parameters. Particularly, the DLPNO variants such as cc-G4-T-DLPNO are applicable to fairly large molecules at a modest computational cost, as is (for a reduced range of elements) a different variant using MP2-F12/cc-pVTZ-F12 for the MP2 component.

Unraveling spectral shapes of adventitious carbon on gold using a time-resolved high-resolution X-ray photoelectron spectroscopy and principal component analysis

To extract chemical information reliably, we propose a new data processing method based both on the creation of information vectors and on a vector base change. The originality of this method is the combination of the different core XPS peaks, Auger and/or valence bands in a single vector. We show that the measurement of a sample upon its progressive chemical change, such as accumulation of the adventitious carbon, allows us to create a new vector basis using linear algebra and Principal Component Analysis. We demonstrate the application of this method using adventitious carbon films on Au foil utilizing Au 4f, C 1s and O 1s spectral envelopes. This method expands the possibilities of XPS measured chemical environment analysis and is an operator-unbiased solution for materials for which the reference XPS spectra are not available. In this demonstration, the emphasis is placed on identifying changes in a C 1s peak assumed to be adventitious carbon and highlight uncertainties associated with calibrating the binding energy scale using a simple peak model to identify a component C 1s assumed to be saturated hydrocarbon in origin.

Electronic Structure of an Ultrathin Molybdenum Oxide Film

The electronic structure of an ultra-thin molybdenum oxide film obtained by oxidation of molybdenum at an oxygen pressure of 1 Torr and the effect of adsorption of sodium atoms on its electronic structure are studied by photoelectron spectroscopy. Photoemission spectra from the valence band and core levels of O 2s, Mo 3d, Mo 3p, and Na 1p are studied upon synchrotron excitation in the photon energy range 80–600 eV. It is shown that in the formed oxide film, molybdenum is in two states: Mo6+ and Mo4+. On the surface of the oxide, oxygen is induced both in the composition of the oxides and in hydroxyl. It was shown that MoO3 is formed on the surface, and MoO2 at a distance from the surface. The deposition of Na atoms leads to intercalation of the molybdenum oxide layer.

Band offset studies in Cr2O3/Ti0.02Cr1.98O3 bilayer film using photoelectron spectroscopy

Realization of oxide/oxide semiconductor heterostructures based high performance electronic/optoelectronic/magneto-optical devices warrants in-depth understanding of energy band alignment at their interface. Herein, we report energy band alignment in pulsed laser ablated Cr2O3/Ti0.02Cr1.98O3 bilayer film from the knowledge of core level energies and valence band maxima positions in the corresponding Cr2O3 and Ti0.02Cr1.98O3 single layer films and their respective shifts in Cr2O3/Ti0.02Cr1.98O3 bilayer film. A type II (staggered) band alignment was identified, with the valence band offset and conduction band offset equals to 1.40eV and 1.13eV, respectively. This investigation will provide further insights into the fundamental properties of Cr2O3/Ti0.02Cr1.98O3 heterojunction, which can be effectively utilized for the design, modelling and analysis of optoelectronic/magneto-optical devices.

Gold Nanoparticles Adsorbed on Tungsten: Effect of Sodium Atom Deposition and Heating

We studied the electronic structure of gold nanoparticles deposited on a tungsten surface before and after the deposition of sodium atoms with subsequent heating at T = 630 K by in situ photoelectron spectroscopy in ultrahigh vacuum. The photoemission spectra from the valence band and core levels of Au 4f and Na 2p were studied upon synchrotron excitation in the photon energy range of 80–600 eV. The changes in the spectra of the valence band and core levels of Au 4f and Na 2p are associated with a change in the surface topography caused by the deposition of sodium atoms and heating, which led to an increase in the surface area by several times. The surface topography and cathodoluminescence of a layer of gold nanoparticles deposited on a tungsten surface are studied.

Hard X‐ray photoelectron spectroscopy study of core level shifts at buried GaP/Si(001) interfaces

We present a study of buried GaP/Si(001) heterointerfaces by hard X‐ray photoelectron spectroscopy. Well‐defined thin (4–50 nm) GaP films were grown on Si(001) substrates with 2° miscut orientations by metalorganic vapor phase epitaxy. Core level photoelectron intensities and valence band spectra were measured on heterostructures as well as on the corresponding reference (bulk) substrates. Detailed analysis of core level peaks revealed line broadening and energetic shifts. Valence band offsets were derived for the films with different thickness. Based on the observed variation of the valence band offsets with the GaP film thickness and on the experimental evidence of line broadening, the existence of charge displacement at the GaP/Si(001) interface is suggested.

Charge correlation in V2OPO4 probed by hard x-ray photoemission spectroscopy

Electronic properties of V$_2$OPO$_4$ have been investigated by means of hard x-ray photoemission spectroscopy (HAXPES) and subsequent theoretical calculations. The V 1$s$ and 2$p$ HAXPES spectra are consistent with the charge ordering of V$^{2+}$ and V$^{3+}$. The binding energy difference between the V$^{2+}$ and V$^{3+}$ components is unexpectedly large indicating large bonding-antibonding splitting between them in the final states of core level photoemission. The V 1$s$ HAXPES spectrum exhibits a charge transfer satellite which can be analyzed by configuration interaction calculations on a V$_2$O$_9$ cluster. The V 3$d$ spectral weight near the Fermi level is assigned to the 3$d$ $t_{2g}$ orbitals of the V$^{2+}$ site. The broad V 3$d$ spectral distribution is consistent with the strong hybridization between V$^{2+}$ and V$^{3+}$ in the ground state. The core level and valence band HAXPES results indicate substantial charge transfer from the V$^{2+}$ site to the V$^{3+}$ site.7 figure

Model potential study of Rydberg one-electron spectrum of thallium

Semi-empirical model potential calculations of excitation energies, fine structure splittings, oscillator strengths and line strengths ratios were computed for high Rydberg transitions in principal, sharp and diffuse series of thallium one-electron spectrum with allowance for core–valence electron correlations in core polarization picture. Relativistic effects are fully included through solving Dirac equations and core–valence electron correlation is represented in core polarization picture by appropriate term in model potential optimized to reproduce ionization energies of the system for lowest states of given |nlj〉 symmetry. The dipole-moment operator of transition is modified in accordance. Systematic trends are investigated in line strength ratios and oscillator strengths along the spectral series, the latter showing hydrogen-like behavior for large n* effective quantum number of upper states of transition. The influence of core–valence correlation (core polarization) on line strength ratios and oscillator strengths along the spectral series is studied in detail demonstrating its importance even for transitions to highly excited Rydberg states with n=20. The strong effect of cancellation of positive and negative contributions to transition matrix element was observed for oscillator strengths and line strength ratios in principal series of thallium spectrum.