Soft X-ray Absorption Spectra Research Articles

Sulfur chemical state analysis of diesel emissions of vehicles using X-ray absorption

Soft X-ray absorption spectra of sulfur K edge were measured for diesel exhaust particles from three different vehicles. The X-ray spectra were measured using a synchrotron radiation beamline. The spectra were measured by surface sensitive total electron yield method and bulk sensitive X-ray fluorescence yield method. One vehicle concentrated the S 2− on the surface of the emission particles; the others did not concentrate S 2−. This type of chemical state analysis method is useful for the process analysis such as diesel emissions.

Study on N2-added and B-doped effect in Co/Pd multilayered films using soft x-ray absorption and magnetic circular dichroism

The soft x-ray absorption and magnetic circular dichroism spectra at the Co L2,3-edges of [Co/Pd]20 and [CoB/Pd]20 multilayered films, which were sputter-deposited in Ar gas containing N2 at room temperature, were measured to investigate the electronic and spin states of the films. The macroscopic magnetic properties, such as coercivity and saturation magnetization, and microstructural properties, such as grain size and preferred orientation, were markedly changed for the [Co/Pd]20 film by the B doping and N2 addition. The B doping and/or the addition of N2 gas caused appreciable changes not only in macroscopic magnetic properties but also in microscopic properties, such as the spin angular momentum and orbital angular momentum, throughout the multilayered film. On the other hand, a new spectral feature at the high energy side of the Co L3-edge peak in x-ray absorption spectra was found in only the films with N2 added. Thus, the microscopic properties, such as electronic and spin states, of the Co 3d electronic state for the [Co/Pd]20 and [CoB/Pd]20 films were changed by only the sputtering process with additive N2 gas.

Monolithic, high surface area, three-dimensional GeO2 nanostructures

We report a sol-gel synthesis of monolithic three-dimensional GeO2 nanostructures (aerogels). Transmission electron microscopy shows that these ultralow-density (∼99.2% porous) nanostructures are formed by a continuous network of elongated amorphous GeO2 nanoligaments, ∼15–20nm in diameter and ∼50–200nm in length. As a result, monoliths exhibit a high surface area of ∼60m2g−1. A comparison of soft x-ray absorption spectra at GeL3 and OK edges with theoretical densities of states reveals that the electronic structure of aerogels is similar to that of hexagonal GeO2, indicating a (distorted) tetrahedral environment of Ge atoms in the amorphous nanostructure.

Momentum selectivity and anisotropy effects in the nitrogenK-edge resonant inelastic x-ray scattering from GaN

High-resolution soft x-ray emission and absorption spectra near the N $K$-edge of wurtzite GaN are presented. The experimental data are interpreted in terms of full-potential electronic structure calculations. The absorption spectra, compared with calculations including core hole screening, indicate partial core hole screening in the absorption process. The resonantly excited x-ray emission spectra show pronounced dispersion of spectral structures, which is attributed to effects of momentum conservation in the resonant inelastic x-ray scattering (RIXS) process. In development of GaN based optoelectronics, momentum selectivity in RIXS can be utilized to control development of band structure in GaN nanostructures.

Revelation of Ni magnetic moment in GdNi single crystal by soft X-ray magnetic circular dichroism

Magnetic moment of Ni in GdNi single crystal was studied through the soft X-ray absorption spectra (XAS) and the magnetic circular dichroism (MCD) at the Ni L 2,3-edges and the Gd M 4,5-edges. Our experiment revealed for the first time that the Ni 3d band is not filled completely even at the content of 50 at.% of Gd and the Ni does retain a total magnetic moment coupling antiparallel to that of Gd. This result implied that the Ni in GdNi holds an intrinsic magnetic moment even at 50 at.% of Gd and contradicts the well-known charge transfer model. Further by employing the magneto-optical sum rule, the spin and orbital angular magnetic moment were evaluated and discussed.

Electronic structure of the dilute magnetic semiconductorZn0.90Mn0.10Sobtained by soft x-ray spectroscopy and first-principles calculations

Soft x-ray emission and absorption spectra of the Mn ${L}_{2,3}$-edge were obtained from the diluted magnetic semiconductor ${\mathrm{Zn}}_{0.90}{\mathrm{Mn}}_{0.10}\mathrm{S}$, and interpreted with the aid of first-principles computations of the densities of states. The calculations reveal that the Mn $d$ states near the Fermi energy show interesting spin polarization, and are strongly hybridized with the neighboring S atoms, indicative of the $sp\text{\penalty1000-\hskip0pt}d$ hybridization typical of dilute magnetic semiconductors. Resonant inelastic x-ray measurements reveal a 3-eV inelastic energy loss, which is interpreted as a transition from the Mn valence-band $d$ states to a spin-flip multiplet of the $3{d}^{5}$ ground-state configuration, while a 6-eV loss is attributed to observed charge-transfer effects caused by hybridization between the S $p$ states and the occupied Mn $d$ states located a few eV below the Fermi energy.

Final state effects in VUV and soft X-ray absorption spectra of transition metal oxides and silicate alloys: comparisons between experiment and ab initio calculations

This paper uses X-ray absorption spectroscopy and vacuum ultra-violet spectroscopic ellipsometry to study the electronic structure of high- k transition metal (TM) oxide gate dielectrics. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d * states, which define the optical band gap, including their relationship to the band gap, E g, of the oxide.

Studying 4d–4f transitions in Er using resonant inelastic scattering

The 4d–4f giant resonance in Er 2O 3 is studied using resonant inelastic X-ray scattering, and the results are interpreted within a purely atomic framework. The soft X-ray emission and absorption spectra are measured using tunable synchrotron radiation. The results show significant valency mixing, evidenced by Er 3+, Er 2+, and Er 4+ components to the soft X-ray absorption and resonant emission data. We find strong inelastic resonant scattering and the energy losses are assigned to 4f inner-shell excitations. The results also show resonant mixing of intermediate states that are separated over an energy range of at least 9.3 eV.

Resonant O Kα emission spectroscopy of layered manganate La 1.2Sr 1.8Mn 2O 7

The O Kα soft X-ray emission and absorption spectra (XES and XAS) of La 1.2Sr 1.8Mn 2O 7 were measured at selected excitation energies using synchrotron radiation in order to investigate the electronic structure. The dependence of O Kα XES on excitation energy was observed. It is attributed to the 2p partial density of states of inequivalent oxygen sites. It was found that O(1) atom makes Mn 3d 3 z 2 − r 2 orbital stable.

Valence in the Rydberg/continuum region in molecular inner-shell spectroscopy

The core-to-valence, especially core-to-σ*, excitation is sometimes obscure in molecular soft X-ray absorption spectra. The potential energy curve (PEC) of the core-to-σ* state is repulsive for the bond elongation; on the other hand, the PECs of the core ionized state and the core-to-π* state are bound and the core-to-Rydberg states are almost parallel to the PEC of the core ionized state. Thus, the σ* state can have interactions with the Rydberg and continuum states, and even with the π* state in some cases, around the Franck–Condon region. Using schematic PECs, we have discussed, where the σ* state is located in the 1s photoabsorption spectra of N 2, O 2, CH 3F, and C 2H 2, and how we know evidence for the σ* state in the 1s photoabsorption spectra of CH 4.

X-ray absorption spectroscopy in MnxGe1−x diluted magnetic semiconductor: Experiment and theory

Accurate first-principles calculations of soft x-ray absorption spectra are compared with experimental data obtained for the ion-implanted MnxGe1−x ferromagnetic semiconductor. The well-defined features in the spectra are recognized as a signature of homogeneous Mn dilution within the Ge host, as demonstrated by comparing the Mn spectra in diluted MnGe alloys with other competing Mn–Ge crystalline phases. Moreover, provided that an efficient Mn dilution is achieved, the nature of the semiconducting host is shown to affect only slightly the Mn absorption spectrum, as shown by the similarity of the present results with those for other magnetic semiconductors. Both these findings establish the relevance of ion-implantation in the dilute magnetic semiconductor framework, emphasizing its potential impact in device technology.

Soft x-ray absorption spectra of amorphous carbon

Information about the local structure of amorphous carbon (a-C) films was obtained by comparing soft x-ray absorption spectra in the CK region of sputtered a-C films to reference carbon compounds such as hydrofullerene (C60H36), highly oriented pyrolytic graphite (HOPG), and carbon black. The spectra of the a-C films exhibited fine structures, which consisted of at least different six portions. Comparing the spectral features of the a-C films with the reference compounds and the spectral analysis by discrete variational (DV)-Xα molecular orbital calculations indicated that the fine structures of the a-C films are due to a hybrid of sp2 and sp3 carbon atoms.

Direct observation of the Fe substitution effect on the MCD spectra of the dysprosium iron garnet family

Soft x-ray magnetic circular dichroism (MCD) and absorption spectra (XAS) were measured at the Fe L2,3-edges and the Dy M4,5-edges for a family of dysprosium iron garnets. The Fe ions sit in either Oh symmetry sites or Td symmetry sites in Dy3Fe5O12. Only Fe ions in the Oh sites are substituted with In ions in Dy3Fe4.25In0.75O12 and only Fe ions in the Td sites are substituted with Ga ions in Dy3Fe4.25Ga0.75O12. The XAS spectrum profiles of the three-dysprosium iron garnets are very similar. The MCD signals of both the Fe L2,3-edges and the Dy M4,5-edges of Dy3Fe4.25Ga0.75O12 are similar to that of Dy3Fe5O12, but Dy3Fe4.25In0.75O12 is remarkably different. The results directly show that the spin moment of Fe in the Td sites play a major role in determining the total magnetization of Dy3Fe5O12.

Magnetic circular dichroism at Fe and V L2,3 thresholds of Heusler-type Fe 2− xV 1+ xAl

Soft X-ray absorption (XAS) spectra and their magnetic circular dichroism (MCD) have been studied at the transition-metal L 2,3 thresholds of Heusler-type Fe 2− x V 1+ x Al intermetallic compounds. Satellite structures 5 eV above the main Fe and V L 2,3 XAS peaks are attributed to a singularity in the density of states, and fine structures in the V L 2,3 spectra is ascribed to the multiplets. Observed MCD intensity changes as the bulk magnetization on changing the content x in Fe 2− x V 1+ x Al. Although the observed MCD signals are small, they definitely indicate the anti-ferromagnetic coupling between the Fe and V magnetic moments, as predicted for the magnetic cluster formed around the anti-site defects in the Heusler lattice. The existence of the magnetic cluster may cause the unusual transport properties at low temperatures.

Soft X-ray absorption spectroscopy in liquid environments

Fluorescence yield (FY) soft X-ray absorption (SXA) spectra from electrolytes, aqueous complexes and biomolecules in liquid environment are presented in order to demonstrate the capabilities of a newly designed liquid sample cell which is compatible with ultra high vacuum environments. Changes of the electronic structure are observed in sodium electrolytes as a function of concentration and solvent.

X-Ray Raman Spectroscopy—A New Tool to Study Local Structure of Aromatic Hydrocarbons and Asphaltenes

Polycyclic aromatic hydrocarbons (PAHs) are essential components in numerous natural and man made organic compounds. In asphaltenes, knowledge of the electronic structure of both its aromatic and aliphatic components is of crucial importance, when trying to understand and predict its macroscopic properties. Theoretical studies of complex PAHs have led to simple rules to describe their π electron distribution, but systematic experimental support is still needed. A novel approach to directly probe carbon type in PAHs is described here, namely the technique of X-ray Raman spectroscopy (XRS). Conventional X-ray absorption spectroscopy (XAS) has been used for many years for element specific characterization of local structure and chemistry. However, in the soft X-ray region (relevant for light elements) its application to numerous carbonaceous systems including asphaltenes encounters problems. Most of the difficulties, including surface sensitivity and restricted sample environments, stem from the submicron path lengths of soft X-rays and/or electrons. XRS provides a means for obtaining the information content of soft X-ray absorption spectra while maintaining the experimental benefits of hard X-ray techniques (˜mm path length). In the XRS process an incident photon is inelastically scattered and part of its energy is transferred to excite an inner shell electron into an unoccupied state. Under the dipole approximation, the resulting features are identical to the corresponding XAS spectrum. In the past, the extremely low cross-section of XRS has made this technique impractical, but intense new X-ray facilities and improvements in X-ray optics have helped to advance this unique spectroscopic tool. We have applied XRS to a series of PAHs and several asphaltenes. The results directly support the simple ideas contained in the “sextet-double bond” description of PAHs. Furthermore, we show that XRS can probe the geometrry of aromatic ring systems in asphaltenes, as well as its ratio of aromatic and aliphatic constituents.

Characterization of Carbon Films on the Japanese Smoked Roof Tile “Ibushi-Kawara” by High-Resolution Soft X-ray Spectroscopy

Carbon films on the Japanese smoked roof tile “Ibushi-Kawara” were characterized by high-resolution soft X-ray spectroscopy using synchrotron radiation. By comparing the soft X-ray emission and absorption spectra of Kawara with the reference carbon compounds, it was determined that the carbon films on Kawara consist of mostly carbon-black-like sp2 carbon atoms and that the surface also contains polyethylene-like sp3 carbon atoms. The take-off/incident-angle dependence on the X-ray emission/absorption spectra of Kawara shows that the carbon-black-like sp2 carbon atoms partially form a layer structure oriented parallel to the basal clay plane, and that the degree of orientation of the carbon films is estimated to be 50% that of highly oriented pyrolytic graphite (HOPG). The microstructure of the carbon films on Kawara is one in which half of the carbon-black-like sp2 carbon atoms form layer-structured clusters parallel to the basal clay plane and the rest of the carbon atoms form random-structured clusters which rigidly connect the layer-structured clusters.

Depth selective chemical state analysis of Pb and S in fly ash in municipal solid waste incinerators using X-ray absorption spectroscopy

Soft X-ray absorption spectra of sulfur K edge and lead M 4,5 edges were measured for fly ash samples collected from two types of municipal solid waste incinerators. The X-ray spectra were measured using a synchrotron radiation beam line. The spectra were measured by surface sensitive total electron yield method and bulk sensitive X-ray fluorescence yield method. The instruments to measure the X-ray absorption spectra were an electric current amplifier and a proportional counter. The spectra recorded with these two yield methods were different for one incinerator sample and same for the other incinerator sample. This observation was supported from the X-ray fluorescence quantitative elemental analysis of Pb. One type of incinerator concentrated the lead on the surface of the fly ash particles. The other did not concentrate lead. This type of depth selective chemical state analysis method is useful for the process analysis such as an incinerator.

Angle-resolved soft X-ray emission and absorption spectroscopy of hexagonal boron nitride

Angle-resolved soft X-ray emission and absorption spectra in the BK and NK regions of hexagonal BN were measured using polarized synchrotron radiation. The take-off/incident-angle-dependence on the spectral features in both X-ray emission and absorption is clearly observed. The configuration of the σ and π orbitals, which were calculated using discrete variational (DV)-Xα molecular orbital calculations, explains the angle-resolved soft X-ray emission and absorption spectra. The relative peak intensity of the 394-eV peak in the NK X-ray emission provides useful information about the BN layer ordering.

Soft X-ray absorption spectroscopy study of the effects of Si, Ce, and Mo ion implantation on the passive layer of AISI 304 stainless steel

The chemical modifications introduced in the passive layer of AISI 304 stainless steel after Si, Ce, and Mo ion implantation were investigated and compared with non-implanted steel by soft X-ray absorption spectroscopy. The influence of ion implantation on the passive properties was evaluated by measuring soft X-ray absorption spectra at the Cr, Fe, Ni, Mn and Si 2p in addition to oxygen 1s thresholds. All ion implanted samples show a relative Cr-enrichment at the surface as compared with non-implanted samples. Fe 2p as well as O 1s spectral changes reveal chemical differences in the passive layer as a function of the element ion-implanted.