Edge X-ray Absorption Spectra Research Articles

X-ray absorption spectra from the resonant-convergent first-order polarization propagator approach

The one-photon absorption cross sections of molecular systems have been determined in the high-energy region from the imaginary part of the electric dipole polarizability tensor. In contrast to commonly adopted state-specific methodologies, the complex polarization propagator approach does not require explicit consideration of the excited states and it is open-ended towards multiphoton absorption. It is shown that the electronic relaxation in the core-hole state is well accounted for in the present approach with use of standard density-functional based electronic structure methods. Sample calculations are presented of the $K$-edge x-ray absorption spectra for ${\mathrm{H}}_{2}\mathrm{O}$, CO, ${\mathrm{C}}_{4}{\mathrm{H}}_{4}\mathrm{N}$, and ${\mathrm{C}}_{6}{\mathrm{H}}_{6}$.

Polarization dependence of x-ray absorption spectra ofNaxCoO2: Electronic structure from cluster calculations

In order to shed light on the electronic structure of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, and motivated by recent $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}L$-edge x-ray absorption spectra (XAS) experiments with polarized light, we calculate the electronic spectrum of a ${\mathrm{CoO}}_{6}$ cluster including all interactions between $3d$ orbitals. We obtain the ground state for two electronic occupations in the cluster that correspond nominally to all O in the ${\mathrm{O}}^{\ensuremath{-}2}$ oxidation state, and ${\mathrm{Co}}^{+3}$ or ${\mathrm{Co}}^{+4}$. Then, all excited states obtained by promotion of a $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}2p$ electron to a $3d$ electron, and the corresponding matrix elements are calculated. A fit of the observed experimental spectra is good and points out a large $\mathrm{Co}\text{\ensuremath{-}}\mathrm{O}$ covalency and cubic crystal field effects, that result in low spin $\mathrm{Co}\phantom{\rule{0.3em}{0ex}}3d$ configurations. Our results indicate that the effective hopping between different Co atoms plays a major role in determining the symmetry of the ground state in the lattice. Remaining quantitative discrepancies with the XAS experiments are expected to come from composition effects of itineracy in the ground and excited states.

Molecular and electronic structure in NaCl electrolytes of varying concentration: Identification of spectral fingerprints

Near edge x-ray absorption spectra at the Na K edge in aqueous NaCl electrolytes are presented as a function of concentration. The spectra are modeled by electronic structure calculations. We find and identify the orbital origin of two spectral fingerprints with sensitivity to the Na+-H2O distance and the Na+-Cl- distance in the electrolyte. Interionic interaction is found to be crucial for the description of the electrolytes at high concentrations.

Direct Observation of Nitrogen Location in Molecular Beam Epitaxy Grown Nitrogen-Doped ZnO

ZnO is a wide band gap, naturally n-type semiconductor with great promise for optoelectronic applications; the main obstacle yet to be overcome is p-type doping. Nitrogen, the most promising candidate currently being pursued as a dopant, has been predicted to preferentially incorporate into the ZnO lattice in the form of a N-2 molecule at an O site when a plasma source is used, leading to compensation rather than p-type doping. We demonstrate this to be incorrect by using N K-edge x-ray absorption spectra and comparing them with first-principles calculations showing that nitrogen, in fact, incorporates substitutionally at O sites where it is expected to act as an acceptor. We also detect the formation of molecular nitrogen upon annealing. These results suggest that effective p-type doping of ZnO with N may be possible only for low-temperature growth processes.

The Electronic Structure of the Hydrated Proton: A Comparative X-ray Absorption Study of Aqueous HCl and NaCl Solutions

The oxygen K edge X-ray absorption spectra of aqueous HCl and NaCl solutions reveal distinct perturbations of the local water molecules by the respective solutes. While the addition of NaCl leads to large spectral changes, the effect of HCl on the observed X-ray absorption spectrum is surprisingly small. Density functional theory calculations suggest that this difference primarily reflects a strong blue shift of the hydrated proton (in either the Eigen (H9O4+) or Zundel (H2O5+) forms) spectrum relative to that of H2O, indicating the tighter binding of electrons in H3O+. This spectral shift counteracts the spectral changes that arise from direct electrostatic perturbation of water molecules in the first solvation shell of Cl-. Consequently, the observed spectral changes effected by HCl addition are minimal compared to those engendered by NaCl. Additionally, these results indicate that the effect of monovalent cations on the nature of the unoccupied orbitals of water molecules in the first solvation shell is negligible, in contrast to the large effects of monovalent anions.

Anomalous behavior of satellite features at the surface and interface in the NiL-edge x-ray absorption spectra

The behavior of satellite features appearing in the Ni $L$-edge x-ray absorption spectrum has been investigated for Cu-capped Ni films on Cu(100) by extracting the interface component of the absorption spectrum. By comparing the results with the surface component, it was revealed that the satellite feature at $\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the ${L}_{\mathrm{III}}$ peak is suppressed only at the surface. This behavior was well reproduced by a one-electron multiple-scattering calculation. On the other hand, an enhancement in the $3\text{\ensuremath{-}}\mathrm{eV}$ satellite was observed at the surface, associated with an increase in the $d$-hole number. This is interpreted as an increase in the $3{d}^{8}$ configuration in the ground state.

Lattice effects in the ferromagnetic insulating phase of manganites

The origin of the ferromagnetic insulating (FMI) phase observed at low temperature in a large variety of manganites is addressed by using x-ray absorption spectroscopy. The variations in the Mn ${L}_{2,3}$ and O $K$-edge x-ray absorption and circular dichroism spectra as a function of temperature are consistent with a carrier localization and localized lattice distortions associated with this phase.

Ultrafast selected energy x-ray absorption spectroscopy investigations of Ni and Zn species

The results of ultrafast selected energy x-ray absorption spectroscopy (USEXAS) investigations of Ni and Zn species are presented. The USEXAS measurements described here employed characteristic x-ray radiation of L(alpha) and L(beta) from an ultrafast laser-driven W x-ray target to probe the K absorption edges of Ni and Zn, respectively. Static x-ray absorption edge spectra of six Ni and Zn species in either solid or solution form were obtained. Simulations of near-edge x-ray absorption spectra of these Ni and Zn species were carried out with FEFF. The results of USEXAS measurements were in general agreement with the theoretically simulated spectra and those measured with synchrotron x-ray radiation.

Reflection mode XAFS at the Ti K-edge of lithium intercalated TiO2 electrodes

Lithium intercalation and deintercalation in crystalline TiO2-electrodes (rutile and anatase modifications) was investigated ex situ using grazing incidence reflection mode x-ray absorption spectroscopy in a specialized cell which permits the removal of the electrolyte and the subsequent measurement of grazing incidence x-ray absorption spectra under a protecting noble gas atmosphere. Small changes observed in near edge x-ray absorption spectra indicated that the Ti4+ ions are reduced by the electrochemical lithiation, depending on the TiO2 modification, Ti3.5+ states as well as Ti3+ states are observed for rutile and anatase, respectively. Angle dependent experiments reveal that the intercalation is not complete for rutile, while anatase seems to be fully intercalated. In addition, a certain amount of Li remains in the anatase electrode after a complete intercalation – deintercalation cycle. The results suggest that Li-ions are generally accumulated at the electrode/electrolyte interface during both intercalation and deintercalation.

Electrochemical lithium intercalation into transition metal oxides

Lithium intercalation and deintercalation in amorphous MoO3-electrodes was investigated in situ using grazing incidence reflection mode x-ray absorption spectroscopy. Small changes observed in the near edge x-ray absorption spectra indicated that the Mo6+ ions are reduced by the electrochemical lithiation, leading to a formal Mo5+ oxidation state. Angle dependent experiments reveal that the intercalation is not complete for the applied conditions, i.e. the Li-ions are not homogeneously distributed in the entire volume of the amorphous working electrode. In addition, a certain amount of Li remains in the Mo-oxide electrode after a complete intercalation/deintercalation cycle. The results suggest that Li ions are accumulated at the electrode/electrolyte interface during both intercalation and deintercalation.

The electronic structures of crystalline and aqueous solutions of NaBr and NaBrO3 using in-situ Na K and Br L edge x-ray absorption spectroscopy

The electronic structures of crystalline and aqueous solutions of NaBr and NaBrO3 were studied using in-situ Na K and Br L edge x-ray absorption spectra measured under ambient conditions. Analyses of Na K-edge x-ray absorption spectra show that NaBr and NaBrO3 strongly dissociate in aqueous solution and form Na+ hydration structures, [Na(H2O)n]+ (n = 0–6). However, the size of the anions significantly affects the formation of the Na+ hydration structures in aqueous solutions. The ligand field potential (10 Dq) observed in the Br L-edge x-ray absorption spectra allows in determining their dependence not only on the coordination structure between the Br and the first near neighbor but also the second near neighbor in crystalline state, and, the hydration structure and the counter cations in aqueous solutions. DV-Xα molecular-orbital calculations predict that the transitions in the Br L-edge NEXAFS occur from Br 2p to the unoccupied states containing mainly 4d orbitals of Br for both crystalline and aqueous solution. However, 5s and 5p of Br and 3p of Na in addition to the 4d orbitals of neighbor Br for crystalline NaBr and 5s and 5p of Br, 2p of O and 1s of H for aqueous NaBr also contribute to the unoccupied states.

Comparison of the Sn L edge X-ray absorption spectra and the corresponding electronic structure in Sn, SnO, and SnO 2

The Sn L 1,2,3 edge X-ray absorption spectra of Sn metal, SnO, and SnO 2 were measured and the assignment of the absorption features discussed. In Sn metal, the white line appeared at the L 1 edge only. While in SnO and SnO 2, it appeared at all the three L edges. This indicates that in Sn metal, the unoccupied states near the Fermi level are mainly of p symmetry, and the bound excited states in Sn oxides possess p and d symmetry. Two weak peaks were observed before the L 3 white line in Sn oxides, which reveals the s symmetry in the bound excited states. These two features were also observed in Sn metal with much weaker intensity, which reveals the hybridization of the s and p states in the band structure.

Testing the magnetism of polymerized fullerene

We present band structure calculations of rhombohedral ${\mathrm{C}}_{60}$ performed in the local-spin-density approximation. Rhombohedral ${\mathrm{C}}_{60}({\mathrm{R}\mathrm{h}\ensuremath{-}\mathrm{C}}_{60})$ is a two-dimensional polymer of ${\mathrm{C}}_{60}$ with trigonal topology. No magnetic solution exists for ${\mathrm{R}\mathrm{h}\ensuremath{-}\mathrm{C}}_{60}$ and energy bands with different spins are found to be identical and not split. The calculated C $2p$ partial density of states is compared to carbon $K$-edge x-ray emission and absorption spectra and shows good agreement. It is concluded that the rhombohedral distortion of ${\mathrm{C}}_{60}$ itself cannot induce magnetic ordering in the molecular carbon. The result of magnetization measurements performed on the same ${\mathrm{R}\mathrm{h}\ensuremath{-}\mathrm{C}}_{60}$ sample corroborates this conclusion.

Molecular nitrogen implanted in Al 2O 3 by low energy N 2+ ion bombardment

Alumina subjected to low energy (∼0.5 keV) N 2 + ion bombardment stabilizes diatomic nitrogen within its matrix. This is confirmed by near edge X-ray absorption spectra of the N K threshold for the implanted nitrogen species that reveals the typical vibrational structure (with the same vibrational spacing) of molecular gaseous di-nitrogen. The analysis of the N KLL resonant Auger emission spectra of the nitrogen implanted alumina samples is characterized by similar features as those found for N 2 in the gas phase. This reaffirms that the incorporated nitrogen in alumina by low energy ion bombardment is in molecular form. The use of resonant Auger spectroscopy is proposed as a general tool to ascertain the formation of di-nitrogen in implanted phases when the photon bandwidth at the N K edge is not narrow enough to resolve the corresponding vibrational structure by X-ray absorption spectroscopy.

Multiple Scattering Calculations of Bonding and X-ray Absorption Spectroscopy of Manganese Oxides

We present near edge X-ray absorption spectra of manganese oxides at the Mn L2,3, Mn K, and O K edges to investigate the relative sensitivity of the edges to bonding and structure. Collectively, the spectra probe local electronic structure and intermediate range crystal structure. Spin independent full multiple scattering calculations of the Mn K edge give good agreement with data above threshold and qualitatively reproduce the prepeak that is observed for each compound. We show that the apparent prepeak for MnO is not due to p-d hybridization at the Mn atom (in accordance with symmetry principles) or quadrupolar transitions but originates from multiple scattering within the fifth shell. We present spin dependent multiple scattering calculations of the O K edge and show that this edge allows for a more direct description of the 3d states than either the Mn L edge or K edge prepeak, which are complicated by multiplet effects.

Surface oxidation of pyrite under ambient atmospheric and aqueous (pH = 2 to 10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy

The nature of the surface oxidation phase on pyrite, FeS 2, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur L III edge, and iron L II,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS 2 can be detected in the X-ray absorption spectra. In a 1.667 × 10 −3 mol/L Fe 3+ solution with ferric iron present as FeCl 3 in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.

Glasses in the TeO 2–Sb 2O 4 binary system

Both the thermal properties and the local structure of glasses prepared in the TeO 2–Sb 2O 4 binary system were investigated. The increase of the glass transition temperature with the antimony oxide concentration indicates a structural evolution of the glass matrix. Infrared and Te L III edge X-ray absorption spectra revealed that the addition of Sb 2O 4 content to the TeO 2 glass former induces the transformation of the tellurium environment from TeO 4 trigonal bipyramids to TeO 3 trigonal pyramids. Furthermore, 121Sb Mössbauer spectroscopy has shown the presence of a mixed valence state, Sb III and Sb V, whith a proportion that depends on the concentration.

Angle dependence of the O K edge absorption spectra of TiO 2 thin films with preferential texture

O K and Ti L 2,3 edge X-ray absorption spectra have been recorded for TiO 2 thin films (∼100 nm thickness) prepared by ion beam induced CVD at room temperature. The as prepared films were amorphous but they crystallize into the anatase structure of TiO 2 after annealing at T>650 K. In the amorphous films, the magnitude of the crystal field, determined from the evaluation of the spectra, was smaller than in the well crystallized anatase samples. In these latter samples the O K edge spectra show a strong dependence on the angle of incidence of the radiation with respect to the sample surface. This dependence rises some concerns about the use of the O K edge spectra of oxide materials for a universal “fingerprint” characterization of thin films. A preliminary theoretical evaluation of the evolution of the t 2g/e g intensity ratio with the relative orientation of the electric field of the radiation and the surface normal of the samples is carried out. For this evaluation, the texture of the thin film (i.e. the preferential growth of certain crystallographic planes parallel to the surface) and the existence of surface modifications of the environment around the Ti are taken into account.

Pb EXAFS studies on DNA quadruplexes: identification of metal ion binding site.

Nucleic acid quadruplexes are composed of guanine quartets stabilized by specific metal ions. X-ray diffraction can provide high-resolution information on the structure and metal binding properties of quadruplexes, but only if they can be crystallized. NMR can provide detailed information on the solution structure of such quadruplexes but little quantitative data concerning the metal binding site. Here we apply extended X-ray absorption fine structure (EXAFS) measurements to characterize the metal ion binding site, in frozen solution, of the unimolecular quadruplex formed by the thrombin binding aptamer, d(G(2)T(2)G(2)TGTG(2)T(2)G(2)) (TBA), in the presence of Pb(2+) ions. The Pb L(III) -edge X-ray absorption spectrum of this metal-DNA complex is very similar to that we obtain for a Pb(2+)-stabilized quartet system of known structure constructed from a modified guanine nucleoside (G1). The Fourier transforms of the Pb(2+) complexes with both TBA and G1 show a first-shell interaction at about 2.6 A, and a weaker, broader shell at 3.5-4.0 A. Quantitative analysis of the EXAFS data reveals the following: (i) very close agreement between interatomic distances at the metal coordination site for the Pb(2+)-G1 complex determined by EXAFS and by X-ray crystallography; (ii) similarly close agreement between interatomic distances measured by EXAFS for the Pb(2+)-G1 and Pb(2+)-TBA complexes. These results provide strong evidence for binding of the Pb(2+) ion in the region between the two quartets in the Pb(2+)-TBA complex, coordinated to the eight surrounding guanine O6 atoms. The specific binding of Pb(2+) to DNA examined here may be relevant to the genotoxic effects of this environmentally important heavy metal. Furthermore, these results demonstrate the utility of EXAFS as a method for quantitative characterization of specific metal binding sites in nucleic acids in solution.

Spectroscopy Studies of Temperature-Induced Valence Transition on EuNi2(Si1-xGex)2 around Eu 3d–4f, 4d–4f and Ni 2p–3d Excitation Regions

The resonant photoemission spectra and X-ray absorption spectra of temperature-induced valence transition material EuNi 2 (Si 1- x Ge x ) 2 around Eu 3 d –4 f , 4 d –4 f and Ni 2 p –3 d resonant excitation regions have been measured. From the comparison between the resonant photoemission spectra around the 3 d –4 f (bulk sensitive) and 4 d –4 f (surface sensitive) excitation regions, the Eu divalent component originating from the surface is separated from the bulk one. The divalent and the trivalent features of 4 f electrons are obviously distinguished depending on the excitation conditions. It was confirmed that the intensity ratio of the 4 f electronic structures between the Eu divalent and trivalent ions changes as a function of the temperature. The mean valence values estimated from the Eu M -edge X-ray absorption spectra and from the photoemission spectra were rather smaller than those obtained previously from the L -edge X-ray absorption spectra. The reason of the discrepancy is discussed. The spectral features and the resonant behaviors are well explained by a theoretical calculation based on the atomic model except for the strong contribution from the Ni 3 d bands. In the resonant condition of the Eu divalent component, the spin flip satellite beside the 4 f divalent states was observed. The photoemission spectra around the Ni 2 p –3 d excitation region show similar behavior to that of Ni pure metal.