X-ray Emission Spectroscopy Spectra Research Articles

A comparative theoretical study on core-hole excitation spectra of azafullerene and its derivatives

The core-hole excitation spectra-near-edge x-ray absorption spectroscopy (NEXAFS), x-ray emission spectroscopy (XES), and x-ray photoelectron spectroscopy (XPS) shake-up satellites have been simulated at the level of density functional theory for the azafullerene C59N and its derivatives (C59N)(+), C59HN, (C59N)2, and C59N-C60, in which the XPS shake-up satellites were simulated using our developed equivalent core hole Kohn-Sham (ECH-KS) density functional theory approach [B. Gao, Z. Wu, and Y. Luo, J. Chem. Phys. 128, 234704 (2008)] which aims for the study of XPS shake-up satellites of large-scale molecules. Our calculated spectra are generally in good agreement with available experimental results that validates the use of the ECH-KS method in the present work. The nitrogen K-edge NEXAFS, XES, and XPS shake-up satellites spectra in general can be used as fingerprints to distinguish the azafullerene C59N and its different derivatives. Meanwhile, different carbon K-edge spectra could also provide detailed information of (local) electronic structures of different molecules. In particular, a peak (at around 284.5 eV) in the carbon K-edge NEXAFS spectrum of the heterodimer C59N-C60 is confirmed to be related to the electron transfer from the C59N part to the C60 part in this charge-transfer complex.

Ab initio computation on the Fe L-edge X-ray emission spectroscopy of Fe-bearing MgSiO3 perovskite

We systematically investigated the L -edge X-ray emission spectroscopy (XES), a 3 d -to-2 p transition, of Fe 2+ - and Fe 3+ -bearing MgSiO 3 perovskite under high pressure based on the internally consistent LSDA+ U technique combined with the Slater-transition method. The Fe L -edge XES spectra can be used to directly interpolate the distribution of Fe-3 d electrons including the spin states and coordination environments of iron. Our results show that the spin transition from the high-spin state to low-spin state of Fe 2+ and Fe 3+ can be identified easily by the L -edge XES technique. The valence state of Fe (2+ or 3+) can be verified by this, since a shift of the first main peak of Fe 2+ across the spin transition of 2 eV, in good agreement with the experimental value (~1.6 eV), is significantly smaller than that of Fe 3+ of 4 eV. The width of the L -edge XES of Fe 3+ also depends strongly on the substitution sites (Mg or Si), meaning that its coordination environments might also be distinguishable based on the Fe L -edge XES spectra. These strong sensitivities to the Fe’s states suggest that the high-resolution Fe L -edge XES would be a useful experimental technique to investigate Fe-bearing silicate minerals.

The Effects of Magnetic Field Size on the Electronic Structure of Al‐Doped ZnO Thin Films Studied by X‐ray Absorption and Emission Spectroscopy

This study examines Al‐doped ZnO (AZO) transparent conductive thin films prepared on glass substrate using the pulsed laser deposition method with an expanding magnetic field perpendicular to the sample surface. O K‐, Zn L3‐, and Al K‐edge X‐ray absorption near‐edge structure (XANES) and X‐ray emission spectroscopy (XES) were used to investigate the relationship between the effects of magnetic field size and the electronic structure of AZO thin films. Analysis of the XANES spectra showed increasing O 2p states as film resistance decreases, suggesting that the enrichment of the O 2p‐dangling bond along the c‐axis is the main factor affecting the electric performance of AZO thin films. Magnetic field size affects electrons itinerating from Zn atoms to Al sites through O 2p–Zn 3d and O 2p–Al 3sp hybridization and consequently weakens O 2p–Zn 3d hybridization with the downsizing of the crystallite size. XES and XANES spectra of O 2p states at the O K‐edge exhibit that the conduction‐band minimum affects Eg and the valence‐band maximum is nearly unaffected by changes in magnetic field size. Factors affecting the resistance/electric conductivity of AZO thin films are the preferential orientation of (002) along the c‐axis, the thermal/annealing effect accompanied by the changes of magnetic field size, the surface effect, energy bandgap, and the density of unoccupied O 2p‐derived states.

Probing substrate-induced perturbations on the band structure of graphene on Ni(1 1 1) by soft X-ray emission spectroscopy

The influence of substrate-induced perturbations on the band structure of graphene has been investigated by soft X-ray emission spectroscopy (XES) for graphene on Ni(111). The band-dispersion features of graphene on Ni(111) are different from those of HOPG and ‘quasi-freestanding’ graphene (graphene/Cu) because of the strong interfacial interaction. By comparing the XES spectra excited with energy at the π∗ resonance of HOPG, graphene/Cu and graphene/Ni in detail, we find that the spectral shape change can be directly related to the different electronic states hybridization strength of graphene on metal substrates, supplying a feasible way for investigating the graphene–metal bonding strength.

Energy-dispersive X-ray emission spectroscopy using an X-ray free-electron laser in a shot-by-shot mode

The ultrabright femtosecond X-ray pulses provided by X-ray free-electron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this "probe-before-destroy" approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ(1,3) XES spectra of Mn(II) and Mn(2)(III,IV) complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to >100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shot-by-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II.

Electronic structure study of ordering and interfacial interaction in graphene/Cu composites

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.

A multi-crystal wavelength dispersive x-ray spectrometer

A multi-crystal wavelength dispersive hard x-ray spectrometer with high-energy resolution and large solid angle collection is described. The instrument is specifically designed for time-resolved applications of x-ray emission spectroscopy (XES) and x-ray Raman scattering (XRS) at X-ray Free Electron Lasers (XFEL) and synchrotron radiation facilities. It also simplifies resonant inelastic x-ray scattering (RIXS) studies of the whole 2d RIXS plane. The spectrometer is based on the Von Hamos geometry. This dispersive setup enables an XES or XRS spectrum to be measured in a single-shot mode, overcoming the scanning needs of the Rowland circle spectrometers. In conjunction with the XFEL temporal profile and high-flux, it is a powerful tool for studying the dynamics of time-dependent systems. Photo-induced processes and fast catalytic reaction kinetics, ranging from femtoseconds to milliseconds, will be resolvable in a wide array of systems circumventing radiation damage.

Study of N-bridged diiron phthalocyanine relevant to methane oxidation: Insight into oxidation and spin states from high resolution 1s core hole X-ray spectroscopy

μ-Nitrido diiron phthalocyanine [PcFe+3.5NFe+3.5Pc]0 is a highly efficient catalyst, able to oxidize methane under near-ambient conditions. In this work, high resolution X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) were applied to study iron species in the series of μ-nitrido diiron phthalocyanines including initial [PcFe+3.5NFe+3.5Pc]0 and oxidized complexes [PcFeIVNFeIVPc]+PF6 and [PcFeIVNFeIV(Pc+)]2+Br2 as model compounds for the intermediates in the catalytic cycle. These systems contain 3d4 configuration of iron in high oxidation state Fe(IV). XES spectra of Kβ line are sensible to the local iron spin density and show unexpected difference in the spin state between the initial [PcFe+3.5NFe+3.5Pc]0 (LS), one-electron oxidized [PcFeIVNFeIVPc]+PF6 (HS) and two electron oxidized cation radical species [PcFeIVNFeIV(Pc+)]2+Br2 (LS). Fe K-edge XANES spectra were recorded at the fluorescence energy of the main Kβ1,3 line or Kβ′ line in the Kβ emission spectrum. The conclusions of RIXS study corroborated XES data. Our study suggests that the main catalytic pathway of oxidation includes two-electron transformation from the LS initial complex to the LS two–electron oxidized [PcFeIVNFeIV(Pc+)(O)]0 to generate powerful oxidant able to oxidize methane via two-electron process.

Electronic structure of Al-doped ZnO transparent conductive thin films studied by x-ray absorption and emission spectroscopies

This study used O K-, Zn L3-, Zn K-, and Al K-edges x-ray absorption near-edge structure (XANES) and O K-edge x-ray emission spectroscopy (XES) measurements to investigate the electronic structure of transparent Al-doped ZnO (AZO) thin film conductors. The samples were prepared on glass substrates at a low temperature near 77 K by using a standard RF sputtering method. High-purity Ne (5N) was used as the sputtering gas. The crystallography of AZO thin films gradually transformed from the ZnO wurtize structure to an amorphous structure during sample deposition, which suggests the suitability to grow on flexible substrates, eliminating the severe degradation due to fragmentation by repeated bending. The O K- and Zn L3-edges XANES spectra of AZO thin films revealed a decrease in the number of both O 2p and Zn 3d unoccupied states when the pressure of Ne was increased from 5 to 100 mTorr. In contrast, Al K-edges XANES spectra showed that the number of unoccupied states of Al 3p increased in conjunction with the pressure of Ne, indicating an electron transfer from Al to O atoms, and suggesting that Al doping increases the negative effective charge of oxygen ions. XES and XANES spectra of O 2p states at the O K-edge also revealed that Al doping not only raised the conduction-band-minimum, but also increased the valence-band-maximum and the band-gap. The results indicate that the reduction in conductivity of AZO thin films is due to the generation of ionic characters, the increase in band-gap, and the decrease in density of unoccupied states of oxygen.

Spin crossover, structural change, and metallization in NiAs-type FeO at high pressure

We report high-spin to low-spin crossover of iron in NiAs (B8)-type Fe${}_{0.96}$O (FeO hereafter) at high pressure by a combination of in-situ x-ray emission spectroscopy (XES) and x-ray diffraction (XRD) measurements. The XES spectra show a loss of spin in B8-type FeO approximately above 119 GPa at 300 K, consistent with the \ensuremath{\sim}2.5% volume reduction around 120 GPa. The high-temperature XRD study also demonstrates a similar volume decrease at 1560--1780 K under the same pressure condition. While the crystal structure of FeO remains to be B8 type across such volume reduction, the atomic arrangements of Fe and O change from inverse to normal NiAs form with considerable decrease in $c$/$a$ axial ratio. With recent electrical resistance measurements, these suggest that iron spin crossover, inverse-normal structural change, and insulator-metal transition occur concurrently in B8 FeO around 120 GPa.

Site-specific intermolecular interaction in α-phase crystalline films of phthalocyanines studied by soft x-ray emission spectroscopy

The local electronic structures of crystalline and amorphous films of zinc phthalocyanine (ZnPc) and metal-free phthalocyanine (H(2)Pc) have been studied by soft x-ray emission spectroscopy (XES). We found a clear crystalline structure dependence of the elastic-peak shape in the resonant XES spectra. The elastic peaks of both ZnPc and H(2)Pc are found to show an asymmetric shape due to resonant inelastic x-ray scattering (RIXS) at the nitrogen sites for the α-crystalline films, but not for the amorphous films. The observed RIXS feature is ascribed to the charge transfer excitation due to the Raman-active intermolecular interaction, which dominates the excited-electron dynamics in α-crystalline phthalocyanine films.

X-ray emission spectra of 3C-SiC studied by tight-binding model including d orbitals

Highlights ► Tight-binding parameters describing the electronic structure of 3C-SiC are determined. ► The C K and Si L spectra for XES and RIXS are simulated and compared with the experiments. ► A contribution of Si d component to XES is different from that to RIXS. Abstract A tight-binding (TB) model with an spds ∗ basis is proposed for zinc-blende silicon carbide (3C-SiC) on the basis of results obtained from the first-principles pseudopotential calculations. In order to ascertain the reliability of this model, the C K and Si L spectra for the X-ray emission spectroscopy (XES) and the resonant inelastic soft-X-ray scattering (RIXS) were simulated using the model, and were compared with available experimental results. The calculated spectra are in good agreement with the experimentally obtained spectra. All the peaks observed in the C K and Si L spectra are interpreted within the framework of the one-electron approximation. It is shown that the Si d component strongly contributes to the high energy part of the Si L 2,3 XES spectrum.

Electronic structure of lithium amide

The electronic structure of the insulator lithium amide (${\mathrm{LiNH}}_{2}$), which is a lightweight complex hydride being considered as a high-capacity hydrogen storage material, is investigated by N $1s$ soft x-ray emission spectroscopy (XES) and absorption spectroscopy (XAS). The XES and XAS spectra show a band gap between the valence and conduction bands. The valence band in the XES spectrum consists of three peaks, which extend up to $~$$\ensuremath{-}8$ eV from the valence band top. The band calculation within the local-density approximation (LDA) for ${\mathrm{LiNH}}_{2}$shows energetically separated three peaks in the occupied N $2p$ partial density of states (pDOS) and the band gap. The energy distribution of three peaks in the XES spectrum agrees with that in the calculated pDOS except for the peak at the highest binding energy, which is attributed to the strongly hybridized state between N $2p$ and H $1s$. The XES experiment has clarified that the strongly hybridized state with H $1s$ in ${\mathrm{LiNH}}_{2}$is located at binding energy higher than that of the LDA calculation, while the overall feature of the electronic structure of ${\mathrm{LiNH}}_{2}$experimentally obtained by XES and XAS is consistent with the calculated result.

The characterization of Cr secondary oxide phases in ZnO films studied by X-ray spectroscopy and photoemission spectroscopy

X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS) were used to characterize the Cr secondary oxide phases in ZnO films that had been prepared using a co-sputtering method. Analysis of the Cr L 3,2-edge XANES spectra reveals that the intensity of white-line features decreases subtly as the sputtering power increases, indicating that the occupation of Cr 3 d orbitals increases with Cr concentration in (Zn, Cr)O films. The O K-edge spectra show that the intensity of XANES features of (Zn, Cr)O films is lower than those of ZnO film, suggesting enhanced occupation of O 2 p-derived states through O 2 p–Cr 3 d hybridization. The XES and XPS spectra indicate that the line shapes in the valence band of (Zn, Cr)O films are quite different from those of ZnO and that the Cr 2O 3 phase dominates the spinel structure of (Zn, Cr)O films increasingly as the Cr sputtering power is increased. Over all results suggest that the non-ferromagnetic behavior of (Zn, Cr)O films can be attributed to the dominant presence of Cr 2O 3, whereas the bulk comprise phase segregations of Cr 2O 3 and/or ZnCr 2O 4, which results them the most stable TM-doped ZnO material against etching.

Ligand Identification in Titanium Complexes Using X-ray Valence-to-Core Emission Spectroscopy

The identification of ligands in metalloorganic complexes is crucial for understanding many important biological and chemical systems. Nonresonant Kβ valence-to-core X-ray emission spectroscopy (XES) has been demonstrated as a ligand identification technique which is complementary to other spectroscopies, such as X-ray absorption. In this study we show the Kβ valence-to-core XES alongside the Ti K-edge X-ray absorption near edge structure spectra for a series of chemically relevant low-symmetry Ti organometallic complexes. The spectra are modeled using density functional theory calculations. XES spectra are analyzed in terms of the molecular orbitals probed, in order to understand the effects of bond length, bond nature, orbital hybridization, and molecular symmetry on the observed spectral features.

Electronic structure study of the bases in DNA duplexes by in situ photon-in/photon-out soft X-ray spectroscopy

Understanding protein functionality is of fundamental importance in biochemistry. Soft X-ray transitions, where the core-level vacancies are filled by the valence-orbital electrons, give direct information about the chemical bonding. Soft X-ray absorption and emission study of poly(dG) -poly(dC) in aqueous solutions can elucidate the relation between the structure and functionality of proteins. We report the N K-edge soft X-ray absorption (XAS) and resonant soft X-ray emission spectroscopy (XES) to characterize the electronic structure near the Fermi level of DNA duplexes to specify the charge migration mechanism. Since N atoms are included in only bases in DNA duplexes, the XES spectra excited from N 1s to unoccupied states purely extract the electronic orbital features of the bases in DNA. The fact that N atoms in different bonding environments form well-defined structure has been determined. The experimental findings provide the evidence for the charge-hopping and/or charge-transfer effects in understanding of electric conduction in DNA duplexes when electrons pass through the π* states of DNA bases.

Change of Structural Behaviors of Organo-Silane Exposed Graphene Nanoflakes

The electronic structures of graphene nanoflakes (GNFs) exposed to an organo-silane precursor [tetramethylsilane, TMS, Si(CH3)4] were studied using electron field emission (EFE), Raman spectroscopy, X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), and first-principles calculation. The results of XANES, XPS, and Raman spectroscopy indicate that the silyl radical strong covalent bonds were formed in GNFs, which induced local structural relaxations and enhanced sp3 hybridization. Comparison of calculated electronic structure, XANES, and XES spectra of Si-treated GNFs suggests that the Si atom substitutes one 3-fold coordinated C atom in a given graphene layer and relaxes outward to form sp3 bonding with another C atom in the adjacent graphene layer. The EFE measurements show an increase in the turn-on electric field with the increase of the Si content, which suggests an enhancement of the nonmetallic sp3 bonding.

High resolution X-ray emission spectroscopy of water and its assignment based on two structural motifs

We evaluate proposed interpretations of previous X-ray emission spectroscopy (XES) data on liquid water. The split peak in the lone-pair orbital region has been interpreted in terms of either two different structural motifs, tetrahedral and distorted, or as due to core-hole-induced dissociative dynamics; here we add new data on a 1:1 H 2O/D 2O isotopic mixture and additional spectrum simulations including the core-hole-induced dynamics. The XES spectrum of HDO is quite nicely reproduced as the sum of spectra of H 2O and D 2O, which we interpret as that core-hole-induced dynamics contribute only to the peak shape and do not affect the intensity ratio between tetrahedrally coordinated and distorted. We find the simulation-based interpretation of the two lone-pair peaks as being of completely different symmetries, molecular 1b 1 and dissociated 3a 1, difficult to reconcile with the experimental intensities in the 1b 2 and 3a 1 spectral regions. We report extensive theoretical simulations of spectra probing both the distance and velocity quantum distributions of the internal OH stretch; sharp features not associated with the lone-pair, that are seen when the OH stretch is treated as a classical oscillator, become smeared out when the zero-point Franck–Condon profile and momentum distribution in the v = 0 level of the OH stretch are taken into account. This demonstrates that neglecting zero-point motion in simulating XES spectra of water generates artificially sharp structures. XES spectra of 1 M and 4 M hydrochloric acid (HCl) and sodium hydroxide (NaOH) are reported. These spectra indicate that dissociated species most likely can be excluded as the origin of the double 1b 1 peak structure. We thus argue that the experimental observation of two distinct peaks in the lone-pair region is less likely to be explained by an unstructured continuum model of the liquid, but is easily explained within a two-component fluctuating model.

Electronic band structure information of GdN extracted from x-ray absorption and emission spectroscopy

The electronic structure of GdN films grown by pulsed laser deposition has been investigated by soft x-ray absorption (XAS) and x-ray emission spectroscopy (XES) at the N K-edge. Density functional calculations within the local spin density approximation with Hubbard-U corrections of the N p weighted bands and density of states are used to extract band information from the spectra. Gd M4,5 XAS and XES spectra are also presented. The XES-XAS separation is shown to give information on the f-band spin splitting and the XAS line shapes are shown to reflect atomic multiplet effects.

Resonant X-ray emission spectroscopy of liquid water: Novel instrumentation, high resolution, and the “map” approach

Techniques to study the electronic structure of liquids are rare. Most recently, resonant X-ray emission spectroscopy (XES) has been shown to be an extremely versatile spectroscopy to study both occupied and unoccupied electronic states for liquids in thermodynamic equilibrium. However, XES requires high-brilliance soft X-ray synchrotron radiation and poses significant technical challenges to maintain a liquid sample in an ultra-high vacuum environment. Our group has therefore developed and constructed a novel experimental setup for the study of liquids, with the long-term goal of investigating the electronic structure of biological systems in aqueous environments. We have developed a flow-through liquid cell in which the liquid is separated from vacuum by a thin Si 3N 4 or SiC window and which allows a precise control of temperature. This approach has significant advantages compared to static liquids cells used in the past. Furthermore, we have designed a dedicated high-transmission, high-resolution soft X-ray spectrometer. The high transmission makes it possible to measure complete resonant XES “maps” in less than an hour, giving unprecedented detailed insight into the electronic structure of the investigated sample. Using this new equipment we have investigated the electronic structure of liquid water. Furthermore, our XES spectra and maps give information about ultra-fast dissociation on the timescale of the O 1s core hole lifetime, which is strongly affected by the initial state hydrogen bonding configuration.