Main Absorption Edge Research Articles

Sensitivity of Soil Manganese Oxides

The examination of soils by x‐ray absorption near‐edge structure (XANES) spectroscopy can provide valuable clues for determining the structure and composition of Mn oxides. XANES experiments can be conducted at ambient temperatures using field‐moist samples. This feature is attractive in the study of soil Mn because of the well‐documented effects of sample drying on reactivity. The present study was undertaken to determine whether the analysis of field‐moist soils by XANES spectroscopy has an effect on the oxidation state of the Mn. Repeated scans were taken on the same spot using seven medium‐ to high‐Mn soils. In all 10 experiments with untreated soils, the main absorption edge dropped in relative energy in successive scans, indicating reduction of Mn. The change in eV was as high as 1.7 eV after 150 min of exposure time. After a series of scans showing reduction, small movement of the sample in the x‐ray beam and reexamination showed no reduction, indicating that the effect was limited to the x‐ray spot. Modeling the change with Mn(II) was successful and showed increases in reduced Mn between 6 and 20% after three to five repeated scans. The slopes of eV change vs. exposure time were remarkably similar in most experiments, even with large differences in the density of photons between runs. Comparisons of initial moist scans with those of recently air‐dried samples found little difference, suggesting minimal reduction in a single XANES scan under our experimental conditions.

Aluminium coordination in tektites: A XANES study

Al K-edge XANES spectra were recorded for six tektites to obtain information on the local structure around Al. Albite, grossular, and andalusite were used as reference materials for Al in fourfold, sixfold, and five+sixfold coordination, respectively. The spectra of the tektites are very similar, except for the intensity of the main absorption edge. Comparison of the spectral features of the tektite glasses with those of the reference materials indicates that Al is tetrahedrally coordinated.

X-ray absorption fine structure studies of FeS2 cathodes in lithium polymer electrolyte batteries

Abstract We have performed synchrotron X-ray absorption measurements on a series of sealed Li/composite polymer electrolyte (CPE)/FeS 2 cells charged or discharged to various potentials. The Fe K-edge measurements include both the near edge (NEXAFS) and extended (EXAFS) regions. The former provides information on the effective Fe valence while the latter reveals the coordination geometry. Six cells charged and discharged at different conditions were examined. Group A consists of: fully discharged (d-1.1 V), almost fully discharged (d-1.25 V), and partially charged (c-1.85 V) cathodes and group B includes fully charged (c-2.25), almost fully charged (c-2.05 V), and partially discharged (d-1.65 V) cathodes. There appear to be only two distinct Fe environments, one for group A and a dramatically different one for group B. The two different main absorption edge peak shapes observed in groups A and B also reflect these distinct Fe environments. The extended fine structure for group A samples reveals an ordered environment dominated by metallic Fe while the higher Li content group B cathodes are characterized by disorder with only a single Fe–S interatomic distance. Spectral fitting to the experimental data of the d-1.65 cell (in group B) suggests that the compound Li 2 FeS 2 is present, with no evidence of FeS. Original cathode material utilization is estimated to be only around 2/3 of full capacity, based on the amount of residual FeS 2 required for satisfactory spectral fitting. On the other hand, no metallic Fe (within detection limits) remains in the recharged cells.

In Situ X-ray Absorption of a Carbon Monoxide−Iron Porphyrin Adduct Adsorbed on High-Area Carbon in an Aqueous Electrolyte

The electronic and electrochemical properties of a carbon monoxide−FeTMPP adduct (FeTMPP = (5,10,15,20-tetrakis[4-methoxyphenyl]-21H,23H-porphine)iron(II)) adsorbed on Black Pearls (BP) high-area carbon have been examined in situ in 0.05 M H2SO4 by Fe K-edge X-ray absorption near-edge structure (XANES). This adduct was prepared by first polarizing a Teflon bonded electrode, incorporating the high-area FeTMPP/BP material, at a potential at which the adsorbed macrocycle is present in the reduced state, followed by bubbling CO into the electrolyte. The resulting in situ XANES displayed two new preedge features centered at 7112 and 7115 eV, as found for other well-characterized iron macrocycle−CO adducts, as well as a shift of the main absorption edge toward higher values (ca. 1.3 V) indicative of charge transfer from the iron to CO. Dissociation of the adduct could be achieved by polarizing the electrode at more positive potentials to yield the CO-free form of the macrocycle in the ferric state, as has been reported for solution-phase CO−Fe porphyrin adducts in nonaqueous media.

In Situ Real‐Time Pt LIII‐Edge X‐Ray Absorption Spectra of Carbon Monoxide Adsorbed on Platinum Particles Dispersed in High‐Area Carbon in Aqueous Electrolytes

Changes in the Pt LIII‐ edge region of small Pt particles (4 to 5 nm diam) dispersed in a high‐area carbon (XC72‐R) induced by the adsorption of CO, were monitored in situ in aqueous by measuring the fluorescence intensity at fixed x‐ray energies during a voltammetric cycle. This real‐time technique made it possible to detect a small increase in the normalized fluorescence (<0.1) at the Pt LIII main absorption edge (11,564 eV), following electro‐oxidation of CO adsorbed at saturation coverages on the high‐area Pt, derived from a shift in the edge toward lower energies. These in situ findings are in agreement with ex situ spectroscopic measurements of Pt surface core levels for CO adsorbed on Pt single crystals in ultrahigh‐vacuum environments. Also described in this work are applications of this new in situ technique to the study of hydrogen adsorption‐desorption and oxide formation and reduction on such high‐area Pt/XC72‐R electrodes in the same electrolyte.

Copper K-Edge X-ray Absorption Studies of La(2)(-)(x)()K(x)()CuO(4) Superconductors, Ba(4)NaCuO(4)(CO(3))(2), and NdCu(2)O(4).

X-ray near-edge absorption spectroscopy (XANES) measurements are reported at the Cu K-edge for La(2)(-)(x)()K(x)()CuO(4) superconductors with x = 0.1 and 0.2. Comparisons of the data were made with the absorption spectra of a Cu(II) standard (La(2)CuO(4)), a new Cu(III) standard (Ba(4)NaCuO(4)(CO(3))(2)), and a new mixed-valent Cu(II)/Cu(III) phase (NdCu(2)O(4)). There is a consistent increase in the energy of the main Cu absorption edge in the following series: La(2)CuO(4) < La(1.9)K(0.1)CuO(4) < La(1.8)K(0.2)CuO(4) approximately NdCu(2)O(4) < Ba(4)NaCuO(4)(CO(3))(2). This trend can be attributed to an increase in the average Cu valence and to a decrease in bond length and an increase in covalency. The magnitude of the energy shift from Cu(II) to Cu(III) for the main absorption edge is approximately 3 eV. There is no measurable change in the energy of the pre-edge feature irrespective of Cu valence. Furthermore, no new features appear at the edge for samples with formal oxidation state between 2 and 3, even for NdCu(2)O(4), which contains both Cu(II) and Cu(III) in crystallographically distinct sites.

Comparative optical studies of Ce2S3 and Gd2S3 compounds

A detailed comparative study of the optical properties of two rare-earth (RE) sesquisulfide compounds γ−Ce2S3 and γ−Gd2S3, shows that if the two compounds have about the same main absorption edge corresponding to band-to-band S 3p-RE 5d transitions, Ce2S3 exhibits an additional absorption band at lower energies. This band, which is ascribed to the presence of Ce 4f states at the top of the valence band, explains the difference in color between the two compounds.

Surface electronic structure at Si(100)-(2x1)

The surface electronic structure of the clean Si(100)-(2x1) surface at room temperature is studied by high-resolution electron-energy-loss spectroscopy. Main absorption edge is detected at ∼ 0.4 eV, which corresponds to the energy gap of the system, further structures are singled out at 0.8 and 1.25 eV and ascribed to interband electronic transitions between dimer-related levels. The paramount importance of cleanness is addressed, showing and quantifying the effect of the residual gas atmosphere (that can be present in ultrahigh vacuum) on the electronic structure.

Electronic structure of thiometalates [MS 4] n− (M=Mo, W, Re). XANES spectra and SCF-Xα-SW calculations

Sulfur K-edge XANES spectra of PPh 4 + salts of the title species are reported. Assignments for the characteristic positions in the spectra are given and discussed. The energies of the pre-edge peaks and the main absorption edges are used to determine the ‘crystal-field’ splitting energies Δ, i.e. the energy difference between the 2e and 4t 2 levels (1.4 eV for [ReS 4] −, 1.1 eV for [WS 4] 2− and [MoS 4] 2−) and the energies of some frontier orbitals (HOMO-LUMO region). These data are compared with results from Xα calculations, electronic absorption and ESCA spectra.

Calculated electronic properties of tetragonal crystalline Si-Ge alloys: Comparison to amorphous phases.

Calculated densities of states (DOS's) and dielectric functions ${\ensuremath{\epsilon}}_{2}$(\ensuremath{\omega}) of crystalline ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ge}}_{\mathrm{x}}$ alloys (x=0,(1/3, 2) / 3 ,1) with 12 atoms per unit cell and space group P${4}_{3}$${2}_{1}$2 [simple-tetragonal structure with 12 atoms per unit cell (ST-12)] are presented. Our DOS results compare favorably with the experimentally known features of the electronic structure of amorphous alloys. For the optical spectra we find that the shift to smaller energies of the main absorption edge as compared to those of diamond-type crystals and the shrinking of the ``optical gap'' with Ge alloying are quantitatively explained by our model. It is further found that the calculations in the ST-12 structure agree better with the measured data for the amorphous phase in the case of Si than for Ge.

Interband optical absorption in amorphous silicon

The interband optical absorption characteristics of amorphous silicon films prepared by various techniques have been investigated. Above the main absorption edge, the absorption coefficient α can be fitted to the “Tauc” model (αh̵ω)12 = C012 (h̵ω − EG). The experimental (αh̵ω)12 versus ħω plots are generally piecewise linear, with an increase in slope above an energy u. Structure in the “Tauc” plots is correlated with preparation conditions; the experimental results are consistent with a broadening of the density of states distribution at the band edges within a gap defined by Eu. The incorporation of bonded hydrogen into the a-Si network results in compositional disorder and deeper potential fluctuations that widen Eu. The bonded hydrogen increases both extrapolated energy gaps; the blue shift is proportional to the line density of hydrogen atoms over a wide range of bonded hydrogen concentrations. A unified model for the interband absorption edge in a-Si and a-Si : H is presented.

Reactive sputtering of titanium and properties of titanium suboxide films for photochemical applications

The effect of oxygen partial pressure on the deposition rate, crystalline structure and optical absorption of thin film titanium suboxides prepared by the reactive sputtering of a titanium metal target is given. A wide variety of films ranging from metallic through semiconducting to dielectric specimens were deposited in a reproducible manner simply by controlling the oxygen content in the sputtering plasma atmosphere. In addition, polycrystalline stable semiconducting TiO 2 electrodes were deposited onto heated glass substrates. The spectral response was investigated; a main absorption edge of about 410 nm was obtained. The design of a special substrate table with a unique mask changer that allowed for the fabrication of different geometrical film patterns is also given.

Sputtered thin film electrodes for photoelectrochemical cells

Semiconducting TiO2 film photoelectrodes were prepared by dc sputtering from a simple hot pressed TiO2 powder target. To make the target semiconducting, it was first heated in a hydrogen atmosphere. Films so produced on heated glass substrates were polycrystalline and strongly adhered to the substrates. When tested for photoelectrolysis, these films were chemically stable and did not exhibit any photodecomposition. The spectral response was investigated and a main absorption edge at ~400 nm was obtained. The technique appears attractive for inexpensive large scale production of photoelectrodes for solar-hydrogen conversion.

Reactively sputtered TiO 2 electrodes from metallic targets for water electrolysis using solar energy

Polycrystalline thin film semiconducting TiO 2 electrodes were prepared by the reactive sputtering of a metallic Ti target. Films were deposited on heated glass substrates and were fabricated in such a way that an n +/n layered structure was formed. Films were hard, dark and strongly adhered to the substrates. Moreover, films were chemically stable and did not show any decomposition. The spectral response was investigated and a main absorption edge at ≈410 nm was obtained. Dependence of photoresponse on light intensity for low levels of illumination was investigated and found to be approximately linear. The exceptional film quality and the enhanced sputtering rates achieved suggest that the technique could eventually prove to be economically viable for water photoelectrolysis applications.

X-Ray Fe K Absorption Edges of [Fe(CN)6]4- and [Fe(CN)6]3-

The X-ray Fe K absorption edge structures of octahedrally coordinated [Fe(CN)6]4- and [Fe(CN)6]3- complex ions have been accurately measured with a high-resolution two-crystal spectrometer. These two hexacyanides have similar low energy absorption in the main absorption edges. The low energy absorption may be principally attributed to the transition of the Fe 1s electron into the 4t1u antibonding orbital constructed mainly from the π* orbitals of (CN)- ligands. The similarity in shifts of the main absorption peaks of both hexacyanides is understood on the basis of Shulman and Sugano's molecular orbital analysis involving backdonation.

X-Ray Co K Edges of [Co(CN)6]3-, [Co(NO2)6]3- and [Co(NCS)4]2- Complex Ions

The X-Pay Co K absorption edge structures of octahedrally coordinated [Co(CN)6]3- and [Co(NO2)6]3-, and tetrahedrally coordinated [Co(NCS)4]2- are accurately measured with a high-resolution two-crystal spectrometer. These three complex ions each have peculiarly low energy absorption in the main absorption edges contrary to Mitchell-Beeman's idea based on Pauling's hybridization theory. The molecular orbital theory provides a qualitative interpretation suitable for these low energy absorptions. In [Co(CN)6]3- and [Co(NO2)6]3- low energy absorption may be principally attributed to the transition of the Co Is electron into the 4t1u antibonding orbital constructed mainly from the antibonding π* orbitals of ligands. In [Co(NCS)4]2- it may be ascribed to Co 1s–2a1 transition.

Measurement of oxygen K absorption spectra using an electron microprobe

AbstractA technique is outlined which use the electron microprobe to obtain X‐ray absorption spectra, by photon absorption, in the long wavelength region. Spectra are presented for SiO, SiO2, rectorite, talc, vermiculite, V2O5, Cr2O3, Fe2O3, maghemite, Mylar and collodion. For all compounds the main absorption edge is near to 536 eV, which is significantly higher than core binding measured in ESCA studies. The transition metal oxides and organic films show a sharp absorption line near 531 eV in addition to the above edge. This absorption line is very close in energy to the anomalous peak observed in the oxygen K emission spectra when using KAP or other acid phthalate crystals and it seems likely that the two phenomena are associated.

Chemical shifts in strontium compounds by X-ray spectroscopy

The positions of the X-ray K-absorption edge of strontium in various compounds and in aqueous solutions have been measured. With the help of modified Moseley diagram the wavelength of the X-ray K-absorption edge of Strontium in metal has been extrapolated. Chemical shifts of the K-absorption edge with respect to this extrapolated value have been presented. The fine structure has been observed on the high energy side of the main absorption edge both in Rubidium and Strontium compounds. Bond lengths for various compounds have been calculated from the maxima-minima separations. The chemical shifts of fluorescence Kα 1,2 lines with respect to their position in Sr 2+ ion have also been studied. While the edge shift with respect to the edge in free ion is towards the low energy side, the line shift with respect to the line in free ion is towards the high energy side.

Band Structure and Fermi-Surface Properties of Orderedβ-Brass

The band structure of ordered $\ensuremath{\beta}$-brass (${\ensuremath{\beta}}^{\ensuremath{'}}\ensuremath{-}\mathrm{CuZn}$) has been calculated throughout the Brillouin zone by the augmented-plane-wave method. The present band model differs from previous calculations with respect to the position and width of the $\mathrm{Cu}3d$ band. The derived dielectric function ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ and the photoemission spectra agree well with experiments. We find that the main absorption edge as observed in the ${\ensuremath{\epsilon}}_{2}$ trace has contributions from conduction-band transitions as well as transitions from the $\mathrm{Cu}d$ states. The comparison to photoemission results indicates that the calculated $\mathrm{Cu}d$ bands are too narrow, but that their position relative to the Fermi level is correct. The derived Fermi-surface model allows a detailed interpretation of the de Hass-van Alphen (dHvA) data. The present model has no open orbit along 100&gt; for $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\parallel}〈110〉$. This agrees with dHvA as well as magnetoresistance measurements. Four new extremal cross sections have been found and related to previously unexplained dHvA frequencies. In general, the Fermi-surface dimensions agree extremely well with dHvA measurements, and the ratios between the experimental and the calculated cyclotron masses vary between 1.18 and 1.43.

Optical absorption studies of AuCr and AuV alloy systems. (Band structure)

The optical properties of AuCr and AuV alloy systems have been investigated through a photon energy range 0.6 eV to 4 eV using an ellipsometric technique on bulk specimens. In the infrared region nearly Drude type absorptions is observed with collision frequency dependent on both concentration and impurity. The movement of the absorption edge on alloying is found to lie between the predictions of the rigid band model and of Friedel's work. Both parts of the main absorption edge are consistent with the assumption that they are associated with transitions from the d band to the Fermi level. In the case of AuCr extra absorption bands were detected compared with pure Au and these were interpreted in terms of virtual energy levels in the region of 1.2 eV from the Fermi level.