Core Electron Excitation Research Articles

Electronic structure of 2(5H)-thiophenone studied by UPS and soft X-ray spectroscopy

Abstract The electronic structure of 2(5H)-thiophenone in the gas phase was investigated by ultraviolet photoelectron spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C 1s, O 1s, and S 2p edges. All assignments of the experimental results are supported by both ab-initio electron propagator OVGF calculations for the valence photoemission bands and density functional theory (DFT) and relativistic time dependent DFT calculations for the core levels XPS and NEXAFS spectra. Overall good agreement between experiment and theory is observed; this is especially true for core electron excitations which has permitted an unambiguous assignment of the observed spectral features in terms of single-particle excitations to virtual molecular orbitals. The assignment of the valence band spectra based on OVGF calculations, although satisfactory, points to the importance of electron correlations effects that partially break the single particle picture of ionization.

Resonant Raman Auger spectroscopy on transient core-excited Ne ions

Abstract Short-lived core-ionized neon atoms were investigated by measuring under resonant Raman conditions the Auger decay following the excitation of a second core electron into a Rydberg state. Making use of intense and narrow bandwidth x-ray free-electron laser pulses, the photoexcitation spectrum of the femtosecond-lived Ne+1s02s22p6np series was characterized. Energy position and lifetimes of the lower-lying Rydberg states were determined and the final state configurations following the decay of the Ne+1s02s22p63p double-core hole resonance were partially resolved.

Simulating X-ray Absorption Spectroscopy in Challenging Environments: Methodological Insights from Water-Solvated Ammonia and Ammonium Systems.

Core-electron excitations in solvated systems, influenced by solvent geometry and hydrogen bonding, make X-ray absorption spectroscopy (XAS) a valuable tool for assessing solvent-solute interactions. However, calculating XAS spectra with electronic-structure methods has proven challenging due to a delicate interplay between correlation and solvation effects. This study provides a computational procedure for XAS modeling in solvated systems, with water-solvated ammonia and ammonium systems serving as probes. Exploring methodological challenges, we investigate explicit embedding models, specifically the polarizable embedding family, including polarizable density embedding and extended polarizable density embedding. Our linear-response time-dependent density functional theory (LR-TDDFT) XAS calculations reveal the efficiency of this approach, with extended polarizable density embedding emerging as a robust improvement over polarizable density embedding. Contrary to some recent literature, our study challenges the belief that LR-TDDFT cannot accurately describe XAS spectra of ammonia and ammonium solvated in water.

Influence of Selective Carbon 1s Excitation on Auger-Meitner Decay in the ESCA Molecule.

Two-dimensional spectral mapping is used to visualize how resonant Auger-Meitner spectra are influenced by the site of the initial core-electron excitation and the symmetry of the core-excited state in the trifluoroethyl acetate molecule (ESCA). We observe a significant enhancement of electron yield for excitation of the COO 1s → π* and CF3 1s → σ* resonances unlike excitation at resonances involving the CH3 and CH2 sites. The CF3 1s → π* and CF3 1s → σ* resonance spectra are very different from each other, with the latter populating most valence states equally. Two complementary electronic structure calculations for the photoelectron cross section and Auger-Meitner intensity are shown to effectively reproduce the site- and state-selective nature of the resonant enhancement features. The site of the core-electron excitation and the respective final state hole locality increase the sensistivity of the photoelectron signal at specific functional group sites. This showcases resonant Auger-Meitner decay as a potentially powerful tool for selectively probing structural changes at specific functional group sites of polyatomic molecules.

Unveiling Hidden Shake-Up Features in the Uranyl M4-Edge Spectrum.

The M4,5-edge high energy resolution X-ray absorption near-edge structure (HR-XANES) spectra of actinyls offer valuable insights into the electronic structure and bonding properties of heavy-element complexes. To conduct a comprehensive spectral analysis, it is essential to employ computational methods that accurately account for relativistic effects and electron correlation. In this work, we utilize variational relativistic multireference configurational interaction methods to compute and analyze the X-ray M4-edge absorption spectrum of uranyl. By employing these advanced computational techniques, we achieve excellent agreement between the calculated spectral features and experimental observations. Moreover, the calculations unveil significant shake-up features, which arise from the intricate interplay between strongly correlated 3d core-electron and ligand excitations. This research provides important theoretical insights into the spectral characteristics of heavy-element complexes. Furthermore, it establishes the foundation for utilizing M4,5-edge spectroscopy as a means to investigate the chemical activities of such complexes. By leveraging this technique, we can gain a deeper understanding of the bonding behavior and reactivity of heavy-element compounds.

Near-edge x-ray absorption fine structure spectra and specific dissociation of small peptoid molecules.

In this study, the total ion yield near-edge x-ray absorption fine structure spectra of four similar peptoid molecules, which differ in the numbers and positions of methyl groups, were investigated experimentally and theoretically. At each excitation energy, the intensity and branching ratio of each ionic product were measured. At a few resonant excitation energies, a specific dissociation of the C-CO bond at the nitrogen and oxygen K-edges and of the N-CO bond at the carbon K-edge was dominant, which correlated well with the predicted destination antibonding orbitals of the core electron excitation. These specific dissociation mechanisms of small peptoid molecules could provide insights into similar phenomena that occur in peptide molecules.

A message passing neural network for predicting dipole moment dependent core electron excitation spectra

A message-passing neural network using a unit direction vector in addition to molecular graphs as the input satisfying invariance to space-inversion symmetry operations enables prediction of the anisotropic core electron excitation spectra.

X-ray induced fragmentation of fulminic acid, HCNO.

The fragmentation of fulminic acid, HCNO, after excitation and ionization of core electrons was investigated using Auger-electron-photoion coincidence spectroscopy. A considerable degree of site-selectivity is observed. Ionization of the carbon and oxygen 1s electron leads to around 70% CH+ + NO+, while ionization at the central N-atom produces only 37% CH+ + NO+, but preferentially forms O+ + HCN+ and O+ + CN+. The mass-selected Auger-electron spectra show that these fragments are associated with higher binding energy final states. Furthermore, ionization of the C 1s electron leads to a higher propensity for C-H bond fission compared to O 1s ionization. Following resonant Auger-Meitner decay after 1s → 3π excitation, 12 different ionic products are formed. At the C 1s edge, the parent ion HCNO+ is significantly more stable compared to the other two edges, which we also attribute to the higher contribution of final states with low binding energies in the C 1s resonant Auger electron spectra.

Core-Projected Hybrids Fix Systematic Errors in Time-Dependent Density Functional Theory Predicted Core-Electron Excitations.

Linear response time-dependent density functional theory (TDDFT) is widely applied to valence, Rydberg, and charge-transfer excitations but, in its current form, makes large errors for core-electron excitations. This work demonstrates that the admixture of nonlocal exact exchange in atomic core regions significantly improves TDDFT-predicted core excitations. Exact exchange admixture is accomplished using projected hybrid density functional theory [ J. Chem. Theory Comput. 2023, 19, 837-847]. Scalar relativistic TDDFT calculations using core-projected B3LYP accurately model core excitations of second-period elements C-F and third-period elements Si-Cl, without sacrificing performance for the relative shifts of core excitation energies. Predicted K-edge X-ray near absorption edge structure (XANES) of a series of sulfur standards highlight the value of this approach. Core-projected hybrids appear to be a practical solution to TDDFT's limitations for core excitations, in the way that long-range-corrected hybrids are a practical solution to TDDFT's limitations for Rydberg and charge-transfer excitations.

Electronic stopping power of protons in platinum: Direct valence and inner-shell-electron excitations from first-principles calculations

The electronic energy loss of proton in platinum (Pt) is studied through nonadiabatic electron-ion coupling dynamic simulations within time-dependent density functional formalism. We have clarified the counterintuitive deviation from velocity proportionality due to the existence of kink velocity of the $d$ electron excitation for a slow proton moving in metal Pt, whose valence $d$ band indeed extends up above the Fermi energy. We have also quantitatively investigated the involvement of the host core electron excitation in the ion-target interaction, through monitoring the evolution of the bound electron number on a specific orbital of the host atom during the close encounter with the flying incident ion. It is found that the low-lying $5p$, $5s$, and $4f$ configuration excitations play significant roles in determining the profile of the stopping curve around and above the stopping maximum.

Pseudopotential Bethe-Salpeter calculations for shallow-core x-ray absorption near-edge structures: Excitonic effects in α−Al2O3

We present an ab initio description of optical and shallow-core x-ray absorption spectroscopies in a unified formalism based on the pseudopotential plane-wave method at the level of the Bethe-Salpeter equation (BSE) within Green's functions theory. We show that norm-conserving pseudopotentials are reliable and accurate not only for valence, but also for semicore electron excitations. In order to validate our approach, we compare BSE absorption spectra obtained with two different codes: the pseudopotential-based code EXC and the all-electron full-potential code Exciting. We take corundum $\alpha$-Al$_2$O$_3$ as an example, being a prototypical material that presents strong electron-hole interactions for both valence and core electron excitations. We analyze in detail the optical absorption spectrum as well as the Al L$_1$ and L$_{2,3}$ edges in terms of anisotropy, crystal local fields, interference and excitonic effects. We perform a thorough inspection of the origin and localization of the lowest-energy excitons, and conclude highlighting the purely electronic character off the pre-edge of L$_1$ and the dichroic nature of the optical and L$_{23}$ spectra.

Bounding [AnO2]2+ (An = U, Np) covalency by simulated O K-edge and An M-edge X-ray absorption near-edge spectroscopy.

Restricted active space simulations are shown to accurately reproduce and characterise both O K-edge and U M4,5-edge spectra of uranyl in excellent agreement with experimental peak positions and are extended to the Np analogue. Analysis of bonding orbital composition in the ground and O K-edge core-excited states demonstrates that metal contribution is underestimated in the latter. In contrast, An M4/5-edge core-excited states produce bonding orbital compositions significantly more representative of those in the ground state. Quantum Theory of Atoms in Molecules analysis is employed to explain the discrepancy between K- and M-edge data and demonstrates that the location of the core-hole impacts the pattern of electron localisation in core-excited states. An apparent contradiction to this behaviour in neptunyl is rationalised in terms interelectronic repulsion between the unpaired 5f electron and the excited core-electron.

Valley selectivity of soft x-ray excitations of core electrons in two-dimensional transition metal dichalcogenides

Optical properties of semiconducting monolayer transition metal dichalcogenides have received a lot of attention in recent years, following the discovery of the valley selective optical population of either ${\text{K}}_{+}$ or ${\text{K}}_{\ensuremath{-}}$ valleys at the direct band gap, depending on the polarization of the incoming light. We use group theoretical selection rules, as well as ab initio DFT calculations, to investigate whether this valley selectivity effect is also present in x-ray optical transitions from the flat core level of the transition metal atom to the valence and conduction band K valleys. Valley selectivity is predicted for $s, {p}_{1/2}$, and ${p}_{3/2}$ edges in transitions to and from the valence band edges with circularly polarized radiation. Possible novel applications to the diagnostics of valleytronic properties and intervalley dynamics are investigated and the feasibility of ultrafast pump-probe and Kerr rotations experiments with suitable soft-x-ray free-electron laser sources is discussed.

Influence of partially occupied sites on the near-edge structure in [formula omitted]-rhombohedral boron: An X-ray Raman scattering study

A study of the near-edge structure at the K-edge of elemental boron in its β-rhombohedral phase is presented. Momentum-transfer dependent measurements of the B 1s core-electron excitation spectra were performed by X-ray Raman scattering (XRS) spectroscopy. Spectral features were interpreted based on calculations of the XRS spectra. A method to model a system with partially occupied sites (POS) was implemented based on FEFF simulations of XRS spectra. The inclusion of POS in the crystal structure of β-rhombohedral boron in the calculations was essential in order to achieve agreement between simulated and measured spectra. Transitions from the core-level to exo-icosahedron orbitals were found to be sensitive to the presence of partially occupied sites in β-rhombohedral boron. The origin and the momentum-transfer dependence of the spectral features are discussed. Characteristic spectral features from icosahedral units and from icosahedron clusters were identified. Activation of non-dipole transitions at high momentum transfers was detected.

Accurate Core-Excited States via Inclusion of Core Triple Excitations in Similarity-Transformed Equation-of-Motion Theory.

The phenomenon of orbital relaxation upon excitation of core electrons is a major problem in the linear-response treatment of core-hole spectroscopies. Rather than addressing relaxation through direct dynamical correlation of the excited state via equation-of-motion coupled cluster theory (EOMEE-CC), we extend the alternative similarity-transformed equation-of-motion coupled cluster theory (STEOMEE-CC) by including the core-valence separation (CVS) and correlation of triple excitations only within the calculation of core ionization energies. This new method, CVS-STEOMEE-CCSD+cT, significantly improves on CVS-EOMEE-CCSD and unmodified CVS-STEOMEE-CCSD when compared to full CVS-EOM-CCSDT for K-edge core-excitation energies of a set of small molecules. The improvement in both absolute and relative (shifted) peak positions is nearly as good as that for transition-potential coupled cluster (TP-CC), which includes an explicit treatment of orbital relaxation, and CVS-EOMEE-CCSD*, which includes a perturbative treatment of triple excitations.

Spectroscopic study on the valence state of Fe in BiFeO3

Bismuth ferrite is a hot spot in the research of multiferroic materials and its spectral research is of great significance to the research of mechanism, properties, and material characterization. The core electron excitation of Fe in BiFeO3 has been studied, and the spectra of Fe K-edge, L2,3-edge, and M-edge were calculated. Combining free-ion multiple theory and crystal field theory, the spectra were calculated to display the core electronic transition of Fe from the shape and peak of the spectrum and theoretically estimated the proportion of Fe3+ in BiFeO3. At L2,3-edge, the shape of the spectrum is mainly determined by the strong dipole transition of 2p→3d. Especially when the content of Fe3+ is 85% and Fe2+ is 15%, the spectrum calculated is in good agreement with the experimental spectrum, and this result is basically consistent with the proportion of 85%–88% in K-edge.

All-Electron BSE@GW Method for K-Edge Core Electron Excitation Energies.

We present an accurate computational approach to calculate absolute K-edge core electron excitation energies as measured by X-ray absorption spectroscopy. Our approach employs an all-electron Bethe-Salpeter equation (BSE) formalism based on GW quasiparticle energies (BSE@GW) using numeric atom-centered orbitals (NAOs). The BSE@GW method has become an increasingly popular method for the computation of neutral valence excitation energies of molecules. However, it was so far not applied to molecular K-edge excitation energies. We discuss the influence of different numerical approximations on the BSE@GW calculation and employ in our final setup (i) exact numeric algorithms for the frequency integration of the GW self-energy, (ii) G0W0 and BSE starting points with ∼50% of exact exchange, (iii) the Tamm-Dancoff approximation and (iv) relativistic corrections. We study the basis set dependence and convergence with common Gaussian-type orbital and NAO basis sets. We identify the importance of additional spatially confined basis functions as well as of diffuse augmenting basis functions. The accuracy of our BSE@GW method is assessed for a benchmark set of small organic molecules, previously used for benchmarking the equation-of-motion coupled cluster method [Peng et al., J. Chem. Theory Comput., 2015, 11, 4146], as well as the medium-sized dibenzothiophene (DBT) molecule. Our BSE@GW results for absolute excitation energies are in excellent agreement with the experiment, with a mean average error of only 0.63 eV for the benchmark set and with errors <1 eV for the DBT molecule.

A new route to obtain fluorescence X-ray absorption spectra of compounds and to remove the self-absorption induced nonlinearity in the spectra.

A new route to obtain fluorescence X-ray absorption spectra of compounds and to remove the self-absorption induced nonlinearity in the spectra is described. The fluorescent intensity If is linearly proportional to the absorption coefficient μ. For studies of surface structures around an element (κ) the fluorescence detection is often the mode of choice. However, the measurement may suffer from a self-absorption (SA) effect which nonlinearly distorts the spectra. The effect is severe when κ is concentrated or the measurements are carried out in certain geometries. Here, the correlations among emission events in compounds are examined following resonance X-ray core-electron excitation within κ. Under conditions leading to SA, If emitted from κ apparently has a conjugated relationship with the fluorescent intensities simultaneously emitted from other elements (ξ). Normalizing the former (κ) by the latter (ξ) will largely remove SA effects and reduce this nonlinear problem to a tractable linear problem. This does result in a moderate reduction of the spectral amplitude due to the so-called secondary emission from ξ excited by the emission from κ. Nonetheless, the resulting spectra will allow one to accurately determine bond distances and disorder and, in some respects, can be superior to spectra obtained via the absorption channel. For μξ < μκ and grazing incidence geometry, the amplitude reduction can be small and simple normalization is sufficient to restore the spectral integrity with remarkable accuracy. This has been instrumental in unravelling the surface and subsurface structures around cations in amorphous Ga-In-O and Zn-Sn-O films which are otherwise inaccessible due to severe SA effects. This method has also been applied to several samples with μξ ≃ μκ to examine its applicability. For these samples, the amplitude reduction is 12 ± 4% versus their standards for the data measured with the classical 45°/45° geometry. This experimental method is easy to implement. Since If from κ and ξ are measured by the same detector system, it is also superior to other methods in removing systematic errors such as detector system nonlinearity, electronic noise, and some beam instabilities, and in removing spectral imperfections due to, for example, SA effects, diffraction effects and sample inhomogeneity. The distortions resulting from the latter can be severe in the spectra measured in transmission mode.

Bridging nano-optics and condensed matter formalisms in a unified description of inelastic scattering of relativistic electron beams

In the last decades, the blossoming of experimental breakthroughs in the domain of electron energy loss spectroscopy (EELS) has triggered a variety of theoretical developments. Those have to deal with completely different situations, from atomically resolved phonon mapping to electron circular dichroism passing by surface plasmon mapping. All of them rely on very different physical approximations and have not yet been reconciled, despite early attempts to do so. As an effort in that direction, we report on the development of a scalar relativistic quantum electrodynamic (QED) approach of the inelastic scattering of fast electrons. This theory can be adapted to describe all modern EELS experiments, and under the relevant approximations, can be reduced to most of the last EELS theories. In that aim, we present in this paper the state of the art and the basics of scalar relativistic QED relevant to the electron inelastic scattering. We then give a clear relation between the two once antagonist descriptions of the EELS, the retarded dyadic Green function, usually applied to describe photonic excitations and the quasi-static mixed dynamic form factor (MDFF), more adapted to describe core electronic excitations of material. Using the photon propagator in a material, expressed in the relevant gauges, as a tool to understand the interaction between a fast electron and a material, we extend this relation to a newly defined quantity, the relativistic MDFF. The relation between the dyadic Green function and the relativistic MDFF does depend only on the photon propagator and not on the specifics of the particle (here, a fast electron) probing the target. Therefore, it can be adapted to any spectroscopy where a relation between the electromagnetic and electronic properties of a material is needed. We then use this theory to establish two important EELS-related equations. The first one relates the spatially resolved EELS to the imaginary part of the photon propagator and the incoming and outgoing electron beam wavefunction, synthesizing the most common theories developed for analyzing spatially resolved EELS experiments. The second one shows that the evolution of the electron beam density matrix is proportional to the mutual coherence tensor, proving that quite universally, the electromagnetic correlations in the target are imprinted in the coherence properties of the probing electron beam.

Manipulating valence and core electronic excitations of a transition-metal complex using UV/Vis and X-ray cavities.

We demonstrate how optical cavities can be exploited to control both valence- and core-excitations in a prototypical model transition metal complex, ferricyanide ([Fe(iii)(CN)6]3−), in an aqueous environment. The spectroscopic signatures of hybrid light-matter polariton states are revealed in UV/Vis and X-ray absorption, and stimulated X-ray Raman signals. In an UV/Vis cavity, the absorption spectrum exhibits the single-polariton states arising from the cavity photon mode coupling to both resonant and off-resonant valence-excited states. We further show that nonlinear stimulated X-ray Raman signals can selectively probe the bipolariton states via cavity-modified Fe core-excited states. This unveils the correlation between valence polaritons and dressed core-excitations. In an X-ray cavity, core-polaritons are generated and their correlations with the bare valence-excitations appear in the linear and nonlinear X-ray spectra.