K-edge Region Research Articles

Dependence of the X-Ray Photoacoustic Signal Intensity of Pure Copper and Brass on Sample Thickness

The hard X-ray photoacoustic signal intensities (I) of pure copper and brass have been measured in the Cu K-edge region as a function of sample thickness (d) (from 5 µm to 100 µm). It is shown that the photoacoustic signal intensities of pure copper and brass decrease above the sample thickness of 10 µm for each modulation frequency. The absolute values of the power law, d(ln I)/d(ln d), of pure copper and brass depend on modulation frequency.

Resonance photoemission and near edge X-ray absorption fine structure (NEXAFS) of Ni 3Al(111) covered with CO

The adsorption of carbon monoxide (CO) on the (111) surface of the ordered alloy Ni 3Al has been investigated by photoelectron spectroscopy using synchrotron radiation. The valence region spectrum was analyzed as the photon energy changed in the range between 60 and 75 eV. A resonance enhancement of a CO peak (1π) was observed for energy close to the Ni 3p ionization threshold (67–68 eV). This result suggested that CO molecules adsorbed on the (111) crystal face of Ni 3Al are preferentially bonded to Ni atoms. Near edge X-ray absorption fine structure (NEXAFS) spectra of the carbon K-edge region have been studied. The spectra were dominated by resonances arising from a sharp bound state transition to an unoccupied molecular orbital of π symmetry and by a broader σ shape resonance in the continuum. The dependence of these resonance structures on the polarization of the synchrotron light indicated the perpendicular orientation of CO on the surface of Ni 3Al(111).

Dependence of the X-Ray Photoacoustic Signal Intensity of Brass on Sample Thickness and Modulation Frequency

Hard X-ray photoacoustic signal intensities of brass have been measured in the Cu K-edge region as functions of sample thickness d (from 10 µm to 300 µm) and modulation frequency f (from 5 Hz to 50 Hz). The photoacoustic signal intensities are proportional to d-1 for each modulation frequency. The X-ray photoacoustic signal varies as f-1.0 up to a sample thickness of 20 µm, whereas it varies as f-0.5 beyond the sample thickness of 20 µm.

EXAFS investigation of vanadium-phosphorus catalysts with different P/V ratios

Vanadium-phosphorus catalysts with different P/V ratios (0.93–1.28) have been investigated by EXAFS and XANES applied to the K-edge region of vanadium. When looked at from the pre-edge peak, vanadium appeared in the 4+ state after drying, while after calcination at 673 K both 4+ and 5+ states occurred simultaneously on samples with P/V > 1. EXAFS revealed that by changing chemical composition, structural variations occur around vanadium. On dried samples, the coordination number decreased from 5 to 4. On calcined samples, the longest V-O distance increased from I.91 to 1.96 A. The possible influence on the catalytic activity is discussed.

K-edge X-ray absorption spectra of biomimetic oxovanadium coordination compounds

Twenty-seven coordination compounds of VO 3+ and VO 2+ of coordination numbers 4–7, containing biomimetic and related ligands in six different coordination geometries have been investigated, using X-ray absorption spectroscopy (XAS) in the K-edge region of vanadium. In contrast to literature reports on less variable series of complexes, no simple correlations between the energy position of the pre-edge white band (1s→3d) and the ligand electronegativities or the oxidation number of vanadium have been found. The feature of the white band, usually a composite of three transitions, and its energy (4.6–6.2 eV in V(V), 4.3–5.8 eV in V(IV) complexes) is mainly a function of geometrical distortions (vanadium-ligand bond lengths and deviation of the vanadium centre from the plane of the polyhedron). Also, there is no apparent correlation between the relative intensity of the 1s→3d transitions and the size of the coordination cage. The K edge (1s→4p) is between 20 and 23 eV with a second maximum, in most cases, at ≈35 eV. Extra features in the K edge arise for peroxo complexes (12 eV), and some octahedral and pentagonal bipyramidal complexes (17 ev).

XANES spectroscopy sensitivity to small electronic changes Case of carp azidomethemoglobin

Spin states equilibrium of hemoglobin-iron varies with external conditions: pH, allosteric effectors, temperature. The small electronic reorganization of the iron caused by the spin state changes has been detected by X-ray absorption near edge structure (XANES) spectroscopy at room temperature. The iron K-edge region which is sensitive to spin state is located in 7110–7130 eV. Here are presented the 100% high spin and 100% low spin XANES spectra of carp azido ferric hemoglobin.

Polarization of molecular x-ray fluorescence.

Polarization of Cl K\ensuremath{\beta} x-ray fluorescence following selective excitation of gaseous ${\mathrm{CH}}_{3}$CL with synchrotron radiation is reported. The degree of polarization of the fluorescence depends sensitively on the chosen incident excitation energy in the Cl K-edge region. Theoretical considerations indicate that the fluorescence-polarization measurements can provide directly absorption and emission anisotropies, molecular-orbital symmetries, and relative fluorescence transition strengths.

Orientation dependence of the carbon K-edge in the electron energy loss spectra of a potassium-benzenegraphite intercalation compound

Electron energy loss spectra of the carbon K-edge region in graphite and the first-stage potassium-benzene-graphite intercalation compound (GIC) have been measured as a function of the crystal orientation defined by the angle between the crystal c-axis and the incident electron beam direction. The intensity of the carbon 1s → π ∗ transition peak strongly depends on the crystal orientation. The dependence for potassium-benzene-GIC differs from that for graphite. By analysing the orientation dependence of the 1 s → π ∗ transition intensity observed in potassium-benzene-GIC, it is found that the molecular plane of benzene intercalating between the graphite layers is tilted relative to the crystal c-axis.

Minimum-dose electron energy-loss spectroscopy

Parallel-detection electron energy-loss spectrometers have greatly reduced the irradiation doses needed to acquire electron energy-loss spectroscopy (EELS) data. This raises the possibility that changes in chemical bonding due to radiation damage, which usually precede changes incomposition, may be studied by examining alterations in low-loss and core-edge fine structure with increasing dose.The quality of data attainable with low-doses in parallel-detection EELS is illustrated by Fig. 1, which shows the K-edge region for a thin amorphous carbon film. This spectrum was collected in 2sec with a 1pm-diameter probe at 100kV in (microscope) diffraction mode. A 20cm camera length ensured that virtually all scattered electrons passed through the 3mm entrance aperture of the spectrometer. Spectrum dispersion is 3eV/channel and the energy resolution is ∼1eV/channel as the π* peak is clearly resolved. The total incident current, as determined in image mode (no objective aperture) from the viewing-screen meter, was only 40pA. The total dose for this K-edge spectrum was consequently only 6.4e-/Å2.

Double photoexcitation processes at the near K-edge region of Ne, Na and Ar

K-absorption structures were calculated for Ne, Na and Ar within the region of photo-double excitation/ionization. Ionization cross section were calculated using the theory of non-orthogonal orbitals and the energies were obtained via configuration interaction method. Calculated spectra are in good agreement with the experiment. It is shown that to describe photoionization probabilities it suffices to take into account the monopole rearrangement of electron shells. In order to interpret the fine structure of the experimental spectra one must consider angular correlations in the movement of both core and excited electrons as well as the fact that the single and multiple ionization channels open at specific energies Calcul de la structure fine du spectre de photoabsorption en couche K dans le domaine de la photoexcitation simple et de la photoionisation double. Calcul des sections efficaces d'ionisation par la theorie des orbitales non orthogonales et des energies par la methode d'interaction de configurations. Necessite de tenir compte du rearrangement monopolaire des couches electroniques. Interpretation de la structure fine des spectres en considerant la correlation angulaire des electrons de cœur et des electrons excites pendant le mouvement, et le fait que les ionisations simples et doubles possedent des seuils differents

K-Edge XANES of GaP, InP and GaSb

X-ray-absorption near-edge structure (XANES) spectra for crystalline GaP, InP and GaSb were measured for each Ga and P K-edge region. Full multiple scattering calculations are performed. The information on the charge distribution around the X-ray-absorbing atom can be obtained by comparing the experimental XANES spectra with the full multiple scattering calculations. The effective cation charge distributions are predicted to be q= ∼0.33 for GaP, q=0.42 ∼0.5 for InP and q \\lesssim0.26 for GaSb.

K-Edge XANES Spectra of GaAs and ZnSe

X-ray absorption near edge structure (XANES) spectra for crystalline GaAs and ZnSe are measured for each K-edge region. Full multiple scattering calculations are also performed. The information on the charge distribution around the X-ray absorbed atom can be obtained if we compare the experimental XANES spectra with full multiple scattering calculations. In these systems core hole potential is important to analyse the spectra.

X-ray absorption spectroscopy of xanthine oxidase. The molybdenum centres of the functional and the desulpho forms.

X-ray absorption spectra have been recorded for the molybdenum K-edge region of xanthine oxidase. Both the absorption edge and the extended fine structure (e.x.a.f.s.) regions were investigated. Spectra were obtained for samples of the desulpho enzyme as well as for mixtures of this with the active enzyme. The spectrum of the pure active form was then obtained by difference. The desulpho enzyme shows a pronounced step in the absorption edge, of a type previously associated terminal oxygen ligands. In the active enzyme this step has decreased markedly. Satisfactory simulations of the e.x.a.f.s. spectrum of the desulpho enzyme could be obtained by assuming the molybdenum to be bonded to two terminal oxygen atoms (Mo = O about .175 nm), two sulphur atoms (presumably from cysteine residues, Mo-S about .0250 nm) and one sulphur atom (presumably from a methionine residue, Mo-S about 0.290 nm). E.x.a.f.s. of the active enzyme differed appreciably from this. In keeping with earlier proposals [Gutteridge, Tanner & Bray (1978) Biochem. J. 175, 887-897], the spectrum of the active enzyme could be simulated if a sulphur atom at about 0.225 nm (i.e. presumably a terminal sulphur atom) replaced one of the terminal oxygen atoms of the desulpho from, with small changes in the other bond distances. Validity of the interpretative procedures, which involved phase shift and amplitude calculations ab initio, was demonstrated by using low molecular weight compounds of known structure.