1s Absorption Research Articles

Soft X-ray absorption at the O K-edge in rutile-type natural oxides

An X-ray absorption study on O 1s edge was carried out on natural oxides with rutile-type crystal structure – the minerals rutile (TiO2), pyrolusite (MnO2), cassiterite (SnO2) and plattnerite (β-PbO2). Despite the chemical bonding being essentially ionic, the differences in the outer electron shell of cations clearly influence post-edge features of XAS spectra. The implications of site symmetry on coordination environment are emphasized for [MO6] octahedra (with quite regular geometry in cassiterite) and for the isosceles triangle [OM3] that displays either two long plus one shorter (pyrolusite and plattnerite) or two short plus one longer (rutile and cassiterite) M–O distances, with M–O–M bond angles always close to 100° plus twice 130°.Theoretical modelling of XAS spectra was carried out. Details of O 1s absorption spectra like the presence of a single post-edge feature for plattnerite and cassiterite may be interpreted by the separation or mixing between O 2p and M nd plus mixing with M ns and np states. Spectra features are commented in relation to crystal structure details but the energy shift assigned for the O K-edge in cassiterite still lacks a plausible explanation.

Evolution of the density of states at the Fermi level ofBi2−yPbySr2−xLaxCuO6+δandBi2Sr2−xLaxCuO6+δcuprates with hole doping

Bi2Sr2-xLaxCuO6+d and Bi2-yPbySr2-xLaxCuO6+d high-Tc superconductors in a wide doping range from overdoped to heavily underdoped were studied by X-ray absorption and photo-emission spectroscopy. The hole concentration p was determined by an analysis of the Cu L3-absorption edge. Besides the occupied density of states derived from photoemission, the un-occupied density of states was determined from the prepeak of the O K-absorption edge. Both, the occupied as well as the unoccupied density of states reveal the same dependence on hole doping, i.e. a continuous increase with increasing doping in the hole underdoped region and a constant density in the hole overdoped region. By comparing these results of single-layer BSLCO with previous results on single-layer LSCO it could be argued that besides the localized holes on Cu sites the CuO2-planes consist of two types of doped holes, from which the so-called mobile holes determine the intensity of the prepeak of the O 1s absorption edge.

Decay of core excitations in bulk h-BN studied with resonant Auger spectroscopy

Non-radiative decay of core excitations at the B 1s and N 1s absorption edges of bulk hexagonal boron nitride (h-BN) has been studied with resonant Auger spectroscopy. In order to reproduce the bulk properties of the sample while keeping it electrically conducting, we have grown reasonably thick (more than 20 nm) polycrystalline films of h-BN on a Ni(1 1 1) surface by thermal cracking of borazine vapor. The probability of the participator Auger process in the non-radiative decay of the B 1s −1π* and N 1s −1π* excitations has been found to be very high (31%) and rather low (below 0.5%), respectively. The drastic difference between the participator Auger decay probabilities of these two excitations has been explained in terms of different localization of the electrons promoted into the lowest unoccupied π* state on the cationic (B) and anionic (N) sites.

Soft X-ray spectroscopy of diluted magnetic semiconductor Ga 1−xM xN (M = Cr, Mn)

We have performed N 1s–2p soft X-ray emission (SXES) and N 1s absorption (XAS) spectroscopy of recently discovered diluted magnetic semiconductor Ga 1−xM xN (M = Cr, Mn). The SXES spectra of doped GaN shows an additional structure on top of the valence band although the rest of the valence band features do not change compared to non-doped GaN SXES spectrum. The XAS spectra of doped samples also show an additional structure in the band gap of GaN, just below the conduction band minimum. Cr substitution results in larger changes in the unoccupied density of states compared to Mn substitution. These results indicate strong hybridization between N 2p and transition metal 3d electrons in Cr or Mn doped GaN. Comparison of the change in the occupied and unoccupied density of states with band structure calculations for the un-doped and doped compositions indicate that the new states correspond to spin split e g and t 2g states.

Resonant inelastic X-ray scattering of Sr 2CuO 3

We measured resonant inelastic X-ray scattering spectra of one-dimensional Mott insulator Sr 2CuO 3 near the Cu 1s absorption edge ( h ν ∼ 8.999 keV) for the purpose of observing the interband transitions between the occupied bands and the unoccupied bands. The obtained RIXS spectra have shown dispersions depending on the wave number difference Δ k from Δ k = 2.25 π to 3.75 π along the single Cu O chain. Each spectrum shows two main features, one is at around 5.5 eV and the other at around 3.1 eV for Δ k = 3.0 π . The 5.5 eV feature does not show noticeable Δ k dependence and can be interpreted as a charge transfer excitation from the Cu 3d–O 2p antibonding state to the upper Hubbard band (UHB). The feature observed near 3.1 eV for Δ k = 3.0 π shows prominent Δ k dependence. The energy loss is the largest near Δ k = 3.0 π and 5.0 π and the smallest near Δ k = 2.0 π and 4 π . This structure is interpreted as the excitation from the Zhang–Rice singlet to the UHB.

Comprehensive study of resonant inelastic X-ray scattering (RIXS) of one-dimensional SrCuO 2

Resonant inelastic X-ray scattering (RIXS) was measured for an insulating SrCuO 2 with changing the incident photon energy ( h ν ) near the Cu 1s absorption edge (K-edge). Spectral shape and peak positions are found to depend strongly not only on the momentum transfer Δ k but also on h ν . The peak width also was found to depend strongly upon Δ k in a good agreement with the theory.

The hybridized M3d-F2p character of low-energy unoccupied electron states in 3d metal fluorides observed by F 1s absorption

The near-edge fine structure of the F 1s absorption spectra of 3d metal fluorides was studied for the first time with high energy resolution. The spectra of these, the most ionic compounds of the 3d atoms, are analyzed comparing with the F 1s absorption spectrum of the molecular TiF62− anion in solid K2TiF6. The latter spectrum was afore interpreted considering the fluorine spectra of the molecular PF6− anion in a KPF6 crystal and of the gas-phase SF6 molecule. The low-energy empty electron states in the 3d metal fluorides are shown to be formed due to covalent mixing of the metal 3d and fluorine 2p electron states.

Soft X-ray emission study of nano-structured carbon

Electronic density of states (DOS) of nano-structured carbon was investigated by soft X-ray emission and absorption spectroscopies (SXES and XAS) near the C 1s absorption energy, and the C 2p partial DOS was obtained. The SXES and XAS spectra of the nano-structured carbon are different from those of graphite or diamond, but similar to those of amorphous carbon, where the sp 2- and sp 3-configured carbon atoms coexist. From the analysis of the XAS spectra, the portion of the sp 3 local structure in the nano-structured carbon is estimated to be 50–55%.

Resonant inelastic X-ray scattering (RIXS) of SrCuO2

The resonant inelastic X-ray scattering (RIXS) was measured for an insulating SrCuO2 with changing the incident photon energy (hν) near the Cu 1s absorption edge (K-edge). Complex structures and their dependences on the momentum transfer (Δk) and hν were observed. The hν dependence of the RIXS spectra measured for a constant Δk=2.4π has shown a remarkable enhancement near the absorption maximum and a shift of the spectral weight toward larger loss-energy with increasing hν. Also, the energies of the inelastic loss peaks in the RIXS spectra for the fixed excitation at 8.993 keV, i.e. 10eV below the absorption maximum, have shown clear dependence on Δk. These results are in a qualitative agreement with a theoretical prediction.

High resolution F1s absorption spectra of solid fluorides of 3d elements

The near-edge fine structure of the 1s absorption spectrum of fluorine atoms from solid-state fluorides of some elements belonging to the first-row transition series was studied for the first time with a high-energy resolution. The spectra were measured with the method of total electron yield by using the Russian-German beamline of monochromatized synchrotron radiation at the electron storage ring BESSY II. The fine structure in the spectra of these most ionic compounds of the 3d atoms is analyzed from comparison of their spectra with the absorption spectrum of fluorine in TiF 6 2− molecular anions from the K2TiF6 crystal. The latter absorption spectrum is preliminarily identified by its simultaneous consideration with the 2p absorption spectrum of titanium in TiF 6 2− and the spectrum of fluorine in PF 6 − anions from the KPF6 crystal and in free SF6 molecules. The lowest free electronic states are shown to form due to covalent mixing of the 3d states of a metal atom with the 2p states of the nearest fluorine atoms.

Local symmetry breaking in paramagnetic insulating(Al,V)2O3

The local structure of (Al_0.06V_0.94)_2O_3 in the paramagnetic insulating (PI) and antiferromagnetically ordered insulating (AFI) phase has been investigated using hard and soft x-ray absorption techniques. It is shown that: 1) on a local scale, the symmetry of the vanadium sites in both the PI and the AFI phase is the same; and 2) the vanadium 3d - oxygen 2p hybridization, as gauged by the oxygen 1s absorption edge, is the same for both phases, but distinctly different from the paramagnetic metallic phase of pure V_2O_3. These findings can be understood in the context of a recently proposed model which relates the long range monoclinic distortion of the antiferromagnetically ordered state to orbital ordering, if orbital short range order in the PI phase is assumed. The measured anisotropy of the x-ray absorption spectra is discussed in relation to spin-polarized density functional calculations.

Variable linear polarization from an X-ray undulator.

A new X-ray undulator has been designed and constructed which produces linearly polarized X-rays in which the plane of polarization can be oriented to a user selectable angle, from horizontal to vertical. Based on the Apple-II elliptically polarizing undulator (EPU), the undulator rotates the angle of the linear polarization by a simple longitudinal motion of the undulator magnets. Combined with the circular and elliptical polarization capabilities of the EPU operating in the standard mode, this new undulator produces soft X-ray radiation with versatile polarization control. This paper describes the magnetic structure of the device and presents an analysis of the magnetic field with varying undulator parameters. The variable linear polarization capability is then exhibited by measuring the X-ray absorption spectrum of an oriented polytetrafluoroethylene thin film. This experiment, which measures the linear dichroism of the sample at two peaks near the C 1s absorption edge, demonstrates the continuous polarization rotation capabilities of the undulator.

Origin of X-ray photon stimulated desorption of Cl + and Cl 2+ ions from Cl/Si(1 1 1)-(1×1)

In this paper we investigate minority adsorption sites on Cl/Si(1 1 1)-(1×1) with the X-ray standing wave technique. By a combination with X-ray photon stimulated desorption, we show that the coordinates of the desorption-active chlorine minority adsorption site can be determined analytically if standing wave difference spectra are recorded at the chlorine 1s absorption edge. Furthermore, the direct and indirect contributions to the total Cl + and Cl 2+ ion desorption yields above the chlorine K edge can be separated and the ratio of the atomic desorption cross-sections can be estimated.

A Near Edge X-ray Absorption Fine Structure Spectroscopy Investigation of the Structure of Self-Assembled Films of Octadecyltrichlorosilane

Polarization-dependent near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to determine the ordering of octadecyltrichlorosilane (OTS) molecules in self-assembled (SA) films on Si/SiOx. Coverages of adsorbed material for different SA films were determined by integration of the NEXAFS signal due entirely to the C 1s absorption. The structures and coverages of the SA films deposited by chemisorption from solution in toluene were determined for two different deposition conditions: (1) open to the atmosphere and (2) water- and oxygen-free conditions inside a drybox. SA films of OTS deposited under ambient air conditions grew as islands on the substrate and were well-ordered as indicated by a strong polarization-dependence in the NEXAFS spectra. For depositions performed in the drybox, ordered or disordered films grew uniformly over the substrate. Ordered SA films deposited in the drybox exhibited higher average tilt angles and lower coverages of adsorbed material than SA films deposited under ambient air conditions. The structure and coverage are discussed with respect to results from measurements of contact angles of water and ellipsometry.

Room temperature self-assembly of CdSe nanocrystals on SiO 2-coated Si wafer

A simple method for deposition of CdSe nanocrystals to silicon surfaces using self-assembled monolayers is described. CdSe nanocrystals were first synthesized by using sodium dioctyl sulfosuccinate as surfactant in inverse micellar solution. An obvious 1s–1s absorption peak at 527 nm and narrow width of photoluminescence peak at 552 nm implied a narrow size distribution of CdSe nanocrystals. Then these nanocrystals were assembled on SiO 2-coated Si substrate by bifunctional linker molecule (3-mercaptopropyl)-trimethoxysilane (MPS). Auger electron spectroscopy showed that the MPS monolayer has been limited on SiO 2-coated Si substrate. The results of atomic force microscopy analysis and contact-angle measurements confirmed that nanocrystals with uniform sizes were bound to MPS-modified substrate through Cd–S covalent bounding.

Organic chemical differentiation within fossil plant cell walls detected with X-ray spectromicroscopy

Organic matter preserved in cell walls of permineralized plant fossils was analyzed by using scanning transmission X-ray microscopy and spectroscopy at energies near the 1s absorption edge of carbon. Microchemical analyses were performed directly on cellulose acetate peels of the fossils, preserving information on the anatomical distribution of organic materials. Individual tracheid walls in both Eocene and Early Devonian fossils exhibit spatially distinct chemical zoning inherited from original wall biopolymers and cell-wall microstructure. Molecular analysis of submicrometer domains using carbon X-ray absorption near-edge spectroscopy documents the differential distribution of hydroxylated aromatic and alcohol (and/or ether) carbon in the inner and outer regions of tracheid walls. This zonation reflects the deposition of lignin and structural polysaccharides in Devonian plants, indicating biochemical and developmental pathways similar to those of living tracheophytes.

Central atom 1s photoabsorption spectra of nitrogen and phosphorus AX 3 (A=N, P and X=F, H) molecules

The newly commissioned SGM beamline [1] at CSRF (the Canadian Synchrotron Radiation Facility at the Synchrotron Radiation Center in Madison WI) offers excellent performance at the nitrogen 1s absorption edge. We have therefore expanded our ongoing X-ray absorption spectroscopy study of electronic structure and chemical bonding of small molecules to include measurements of the N(1s) absorption edges for comparison to the edges of heavier elements in the same group. Here we present the central atom (N, P) 1s spectra of NF 3, NH 3, PF 3 and PH 3 measured at high resolution with synchrotron radiation. The phosphorus edges were measured with the Canadian DCM beamline at CSRF. The observed features were assigned with the aid of MS-Xα and ab initio calculations. Spectral differences are explained in terms of differences in the electronic structure of these four related molecules. All molecules show a strong central atom (A) core (1s) to 1e * transition and additional weaker features which arise from transitions to other virtual molecular orbitals. Chemical shifts resulting from the molecular field of the substituents are observed. Our NH 3 spectrum agrees with that previously published [2].

Resonant OKα emission spectra of CuGeO 3 single-crystal

The electronic structure of CuGeO 3 has been studied using soft X-ray absorption and emission spectroscopy. The unoccupied and occupied valence states are reflected by O 1s absorption and OKα emission spectra, respectively. An absorption pre-peak appearing at 531 eV in O 1s absorption spectrum indicates the localized nature of the unoccupied O 2p admixture to Cu 3d states. A large band gap is observed in the absorption–emission spectrum. Some changes of spectral profile of the OKα emission spectra are attributed to the existence of the inequivalent oxygen sites.

Quantitative Compositional Mapping of Core−Shell Polymer Microspheres by Soft X-ray Spectromicroscopy

Core−shell polymer microspheres have been prepared by a two-step precipitation polymerization. These microspheres consist of 3.2 μm polydivinylbenzene-55 (DVB55) cores coated with ∼0.4−0.9 μm wide shells composed of poly(DVB55-co-EDMA), a random copolymer of DVB55 and ethylene glycol dimethyl acrylate (EDMA). The chemical composition, core−shell morphology, and porosity of these structured microspheres have been studied quantitatively with scanning transmission X-ray microscopy (STXM) at a spatial resolution of 100 nm. Images recorded at selected energies through the C 1s absorption region were analyzed using singular value decomposition to obtain quantitative maps of the DVB55 and EDMA components. The EDMA concentration in the shell determined by STXM was found to be in good agreement with that predicted from the comonomer composition over the range of 10−90% EDMA. The precision of chemical quantification at high spatial resolution is shown to be adequate for STXM to be useful in guiding the development ...

Origin of size effect on superconductivity in (R 0.8Pr 0.2)Ba 2Cu 3O 7 studied by soft X-ray absorption spectroscopy

O K-edge X-ray absorption near-edge structure (XANES) spectroscopy of (R 0.8Pr 0.2)Ba 2Cu 3O 7 (R=Y, Tm, Dy, Gd, Eu and Sm) has been performed to investigate the dependence of hole distribution on ionic size of the R 3+ ions. Near the O 1s absorption edge, pre-peaks at ∼527.8 and ∼528.5 eV are assigned to transitions into O 2p hole states located in the CuO 3 ribbons and CuO 2 planes, respectively. As deduced from O K-edge X-ray absorption spectra, the hole concentrations in the CuO 2 planes decrease monotonically with increasing ionic size of the rare-earth elements. The present XANES results clearly demonstrate that the suppression of superconductivity in (R 1− x Pr x )Ba 2Cu 3O 7 with ionic radius of the R 3+ ions results predominantly from the hole-depletion effect.