Showing papers on "Extended X-ray absorption fine structure published in 1977"

New method for the calculation of atomic phase shifts: Application to extended x-ray absorption fine structure (EXAFS) in molecules and crystals

Abstract: The scattering of electrons of kinetic energy up to 1000 eV by an atom is of special interest in the understanding of extended x-ray absorption fine-structure (EXAFS) spectra. An important physical feature is the reduction of the exchange and correlation potential as the kinetic energy of the electron increases. This is taken into account by replacing the atom by an electron gas with spatially varying density and calculating the self-energy using the plasmon pole approximation. This results in a set of complex phase shifts which is then applied to the EXAFS problem. Comparison is made with phase shifts extracted from experimental EXAFS spectra and excellent agreement is obtained. Direct comparison of the theoretical and experimental spectra again shows excellent agreement in both the amplitude and the phase. We also analyze the EXAFS spectra by a Fourier-transform technique which first removes the amplitude and phase shift using the calculated result. The importance of a proper choice of zero of energy ${E}_{0}$ is emphasized. We choose ${E}_{0}$ by the requirement that the imaginary part and the absolute value of the Fourier transform should peak at the same distance, thus assuring that the absolute phase is given correctly. Using this procedure the nearest-neighbor distances in ${\mathrm{Br}}_{2}$, Ge${\mathrm{Cl}}_{4}$, and crystalline germanium are determined. In all cases the results are within 0.01 \AA{} of the known distances. Several shells in germanium are also determined, with accuracy of better than 1%. Application of our method to crystalline copper indicates that the outer shells are more seriously affected by multiple-scattering problems and our procedure permits us to discard peaks that are spurious or unreliable. The present determination of the nearest-neighbor distance in copper is found to be in error by 0.014 \AA{}. Results of the application of this method to the determination of the bond lengths of a variety of compounds are summarized.

Photoacoustic technique for determining optical absorption coefficients in solids.

Abstract: An investigation was made of a photoacoustic technique for determining the optical absorption coefficient in solids. A train of laser pulses was passed through the solid, and a piezoelectric transducer attached directly to the sample measured the amplitude of the elastic wave generated by the absorbed radiation. Calibration was performed at a wavelength of known absorption. The sensitivity of the technique was found to be limited to about 1 × 10−5 cm−1 in our samples due to radiation scattered onto the transducer, but the technique is capable of measuring absorption coefficients in the 10−6 cm−1 range using laser powers of about 1 W.

Anisotropic x-ray absorption in layered compounds

Abstract: A study is presented of anisotropy effects in x-ray absorption in the layered compounds of $2H\ensuremath{-}\mathrm{W}{\mathrm{Se}}_{2}$ and $1T\ensuremath{-}\mathrm{T}\mathrm{a}{\mathrm{S}}_{2}$. In the measurements it was essential to separate the thickness effect from the true anisotropy effect which is dependent on the angle between the x-ray polarization and the crystal axes. The Se $K$ edge and the W and Ta $L$ edges were measured. Anisotropy in the white line of Se was found but no anisotropy was discerned in the W and Ta white lines. It is pointed out that x-ray absorption in general, and the extended x-ray absorption fine structure (EXAFS) in particular, have the anisotropy dependence of a second-order tensor and the theoretical expression for the EXAFS anisotropy at the ${L}_{2,3}$ edges is explicitly displayed. The anisotropy of the EXAFS in the Se and W absorption was measured and a good agreement with theory is found. The anisotropy of EXAFS at the ${L}_{2,3}$ edges has the new feature of a cross term between the final $s$ and $d$ states, which permits a determination from the measurements that the average contributions of the final $s$ state is 0.02 of that of the final $d$ states to the total absorption of the W ${L}_{3}$ edge. Finally, only qualitative agreement is obtained between band calculations and the near edge x-ray absorption structure, as expected theoretically.

Coordination of Arsenic Impurities in Amorphous Silicon-Hydrogen Alloys

Abstract: We have determined the coordination of arsenic impurities in amorphous silicon-hydrogen alloys prepared by plasma decomposition of silane-arsine mixtures. From analysis of the extended x-ray absorption fine structure (EXAFS) on the arsenic $K$-shell absorption, we find at low arsenic concentrations that an average of two arsenic atoms in ten are fourfold coordinated with silicon. This is the first direct evidence for the occurrence of substitutional doping in an amorphous semiconductor.

Collision-induced absorption of O2 in the Herzberg continuum

Abstract: Using a one meter normal incidence grating monochromator, the total absorption of radiation for O2 has been reinvestigated between 2000-2500 A. The absorption cross sections of O2 and O4 are then derived from total absorption measurements and compared with other available data. Additional measurements of O2 absorption have been made in the presence of Ar which does not absorb in this spectral region and is chemically inactive with O2. It is found that the absorption of oxygen increases in the presence of Ar, which is interpreted as due to the formation of O2-Ar dimers.

Direct and indirect two-photon processes in layered semiconductors

Abstract: Two-photon absorption measurements have been carried out in the layered compounds GaSe, Pb${\mathrm{I}}_{2}$, and GaS using single-wavelength excitation by a neodymium and a ruby laser. The absorption coefficient for direct interband transitions has been measured in all materials by the nonlinear transmittance method. Nonlinear photoconductivity measurements allowed the determination of a nonlinear absorption coefficient for indirect transitions in GaS and the determination of free-carrier lifetime in all three materials. The experimental results are compared with existing theories.

Plant analyses by X-ray spectrometry II—Elements of atomic number greater than 20

Abstract: X-ray spectrometry can be used to analyse plants for elements of atomic number greater than 20 at concentrations as low as 1 ppm. The most accurate results are obtained by measuring peak intensity, background intensity and the mass absorption coefficient of the sample. The intensity of the Compton scatter by the sample of the Au Lα from the X-ray tube can be used as an indirect measure of the mass absorption coefficient, or this coefficient can be calculated if the concentrations of the major elements are known. Background intensity should be measured on each sample for each element but, if not practicable, e.g. in multi-channel instruments, then a close approximation of this intensity can be calculated from the mass absorption coefficient. Calibration has been made using chemicals added to cellulose and the results obtained for the concentration of manganese, iron, copper and zinc in a series of plant samples and in the standard kale sample are in close agreement with the chemical values.

Analysis of laser calorimetric data

Abstract: Adiabatic laser calorimetry, which is the most widely used method for studying the absorption coefficients of low-loss materials, can be adapted to study both the bulk and surface absorption by using a long rod sample geometry. In the limiting case of small heat losses, calculations of the thermal rise curves obtained in laser calorimetry indicate that two regions of constant slope can be expected. The first of these can be identified with the bulk absorption coefficient only and the second with the sum of the surface and bulk absorptions. Experimental data illustrating this two-slope behavior are presented.

Study of Ni(II) coordination in aqueous solution by EXAFS analysis

Abstract: Extended x‐ray absorption fine structure (EXAFS) spectroscopy has been applied to the study of water coordination of the Ni(II) ion in aqueous solution. High resolution x‐ray spectra were measured for Ni(NO3) ⋅6H2O salt and its 0.1M aqueous solution using a synchrotron radiation source. Analysis of the spectra by Fourier transform reveals first sphere coordination in agreement with x‐ray diffraction results. In addition, a second coordination sphere is detected and a tentative distance from the Ni(II) ion to the second sphere water molecules of 3.2±0.1 A is measured.

Frequency modulated X-ray diffraction I. Determination of partial structure factors

Abstract: It is shown that when the frequency of incident X-rays is tuned just below an absorption edge of a particular element in a multi-element sample, the derivative of the scattered intensity with respect to the frequency of the X-rays is dominated by the interference of X-rays about this element. Frequency modulated X-ray diffraction should permit certain partial structure factors to be determined simply with a sensitivity orders of magnitude greater than techniques used previously. This technique should yield at least the same information that X-ray absorption (EXAFS) techniques are supposed to provide, but with fewer complications.

Structural determinations of liquid semiconductors using extended X-ray absorption fine structure

Abstract: The extended fine structure in the X-ray absorption coefficient is dominated by the interference of the photoelectron scattered by atoms in the immediate neighbourhood of the atom which absorbs the X-ray photon and thus can provide structural information about ordered or disordered systems. In this paper it is demonstrated that Extended X-Ray Absorption Fine Structure (EXAFS) measurements can be made on liquid systems at high temperatures. The technique is illustrated with results for As2Se3 in the liquid and amorphous states for temperatures between 100 and 773 K. A Fourier analysis of the EXAFS data reveals that a major structural rearrangement does not occur in the nearest neighbour shell when As2Se3 is melted. However, small structural changes do occur at the melting point which, within the limitations of the present data, suggest a slight increase in the nearest neighbour As–Se distance, a decrease in the number of nearest neighbours, and a decrease in the nearest neighbour disorder term σ12.

Adsorbate and Substrate Characterization Using Extended X-Ray Absorption Fine Structure

Abstract: We illustrate the usefulness of the extended x-ray absorption fine-structure effect for determining the chemical state, structure, and location of adsorbed species by a study of 0.2 monolayers of bromine adsorbed on graphite at room temperature. Bromine adsorbs as molecules with one atom fixed above the basal-plane hexagonal site while the molecular axis is free to flop around.

Residual absorption in infrared materials

Abstract: The sources of the residual absorption of ir materials in their transparent region between 2μm and 10μm are reviewed with emphasis upon recent developments. On the longer wavelength side of this spectral region, many insulating crystals are limited by intrinsic multiphonon absorption, and, on the shorter wavelength side, all insulating crystals are limited by extrinsic effects associated with impurities, defects, and surfaces. During the last few years, the general character of the intrinsic multiphonon absorption has become sufficiently well understood so that its frequency and temperature dependence can be accounted for in both ionic and covalent crystals. The nature of extrinsic absorption is more complicated as it arises from a number of sources. In some cases, the measured absorption coefficient can be attributed with some certainty to a specific origin. In others, especially for the ionic crystals at shorter wavelengths, the origin of the extrinsic residual absorption is not known and may be limited by the experimental measurement techniques, at least in some of the better crystals.

Phase-shift calculations for extended X-ray absorption fine structure (EXAFS) and the fine structure of As2O3

Abstract: Theoretical calculations are presented of three factors involved in Extended X-ray Absorption Fine Structure (EXAFS). It is found that multiple scattering within the atom gives rise to characteristic structure of the back-scattering amplitude. The back-scattering is not sensitive to charge and alters little with small changes in atomic number (Z), but exhibits a wide variety of shapes with large changes of 2. The back-scattering phase-shifts also show a trend similar to that of the amplitude. A comparison of the ionized atom and neutral atom Z=1 phase-shift shows a constant shift of the phase and little change in its k- dependence. Experimental measurements of the fine-structure of cubic and glassy As, O, are reported and are discussed in terms of the preceding calculations. It is found that As, O, molecules do not exist as a major constituent of the glass.

Excited state absorption of 1,3,3,1′,3′,3′-hexamethylindotricarbocyanine Iodide: A quantitative study by ultrafast absorption spectroscopy

Abstract: A quantitative investigation of HITC dissolved in methanol has been made using the method of ultrafast absorption spectroscopy with a streak camera. Samples were excited by picosecond pulses of a mode-locked ruby laser. Analysis of time-resolved spectra yielded non-exponential decay kinetics consisting of a fast (τ variable) and a slow (τ=1.13±0.08 ns) component. The excited state absorption spectrum has its maximum at 493 nm and shoulders at 415 and 540 nm. The excited state absorption cross section was determined by simulataneous measurement of the bleaching of ground state absorption taking polarization of excitation and probe light and excited state absorption at the laser wavelength into account. A value of σ1 (493 nm)=1.0·10−16 cm2 was found.

Photoluminescence and Optical Properties of Ge 1− x Se x Glasses

Abstract: The photoluminescence spectra in Ge1-xSex glasses with a composition of 0.5≤x≤0.9 along with the optical absorption spectra have been measured. The position of the emission peak is almost independent of the Se composition. The emission intensity, however, is most intense for the composition Ge–Se2, indicating the dominant role of GeSe2 stoichiometric structure of Ge–Se glasses on the radiative recombination. The annealing of samples causes a reduction in the energy range of the tail states and thus an increase in the optical energy gap together with an enhancement of the emission intensity. An absorption band is found in the specral range corresponding to the photoluminescence. The absorption coefficient of the absorption band has a close relation with the emission intensity. A model which is partly based on the recombination edge model is proposed. The localized states near the midgap which act as recombination centers are presumed to be due to structural defects associated with GeSe4 molecules.

Angular dependence of polarization-related laser-plasma absorption processes

Abstract: We have measured the incidence-angle dependence of the absorption of p-polarized and s-polarized laser radiation by dense plasmas. A significant absorption peak at nonnormal angles of incidence is observed which is attributed to resonance absorption.

Structure and Polarized Absorption Spectra of Mn3+-substituted Andalusites (Viridines)

Abstract: To contribute to the knowledge of the crystallochemical behavior and spectroscopic features of the 3d4-configurated ion Mn 3+ in silicate structures, we studied Mn 3+substituted andalusites, i.e., viridines, using single-crystal structure refinement (basis: andalusite structure [1], S.G.: Pnnm) and by recently developed methods for absorption spectroscopy on microcrystals [2]. The samples used are characterized and some of the results are summarized in Table 1; the polarized spectra are shown in Figure 1. The structural refinements (Vi-l: R = 2.2%; Vi-2: R = 3 . 7 % ) reveal that Mn 3+ (and Fe 3 +) is incorporated in the M 1-octahedra of the structure, while the five-coordinated M2-sites do not contain significant amounts of transition metal ions. Increasing substitution of Mn 3+ for A1 in M1 causes the viridine lattice constants and the elongation along z of the Ml-octahedra, already present in pure Al-andahisite [1], to increase (Table 1). The octahedral distances (Table 1) show that the point symmetry of Mn 3 § in viridine M 1-octahedra is C2v, but may be approximated by D4h. At present, two models, (a) and (b), can be suggested for the assignment of the observed absorption bands. These models are based on the following considerations: 1) Comparison of the spectra of the synthetic, iron-free viridines with those of the natural iron-bearing viridines shows that the latter ones contain several weak bands missing in the first ones (26 800, 23 300, 22300, 20 800cm i). 2) Fe ar spin-forbidden transitions are expected to occure at these energy values on the basis of present knowledge about such transitions in silicates [3-6]. 3) Crystal field theory predicts three spin-allowed dd-transitions in Mn 3 § in elongated octahedra, point group D4h, I: 5Blg~SAlg, II: SBlg--+SBzg, and III: 5Blg~SEg. On this basis, the bands characteristic for Mn 3 + in natural (N) and synthetic (S) viridines are identified as follows: 15 000 cm1 (N, S): in both models, (a) and (b), transition I; 21 900 era-1 (S) and 23 400+21 500cm -1 (N): in case of model (a), transitions III and II (unresolved in S); in model (b) : 21 900 c m a (S) and 21 500 cm1 (N), transition II and 29 800cm-1 (S) and 28 500cm-1 (N), transition III. Arguments 1 and 2, and the fact that the 21 900-cm1 band in synthetic viridines does not show shoulders, favor model (b). Because the weak bands at 19800 and 18100cm -1 are present in both the synthetic and natural material, they can only be interpreted as spin-forbidden dd-transitions in Mn 3+ and may be assigned to transitions between low-symmetry split terms derived from the 5Eg~3Tlg(H) transition in cubic fields. Because the energy of transition I depends on the Jahn-Teller splitting of the 5Eg ground state in cubic symmetry of Mn 3§ the high energy shift of the 15 000-cm1 band on increasing Mn 3+ substitution (cf. Fig. 1 B and x values in Table 1) is well in accordance with the increasing elongation of M1 as found in the structure refinements. The crystal field stabilization energies (CFSE) of Mn a+ in viridines (Table 1), calculated with both assignment models (a) and (b), are relatively high when compared with those of Mn 3+ in other Mn 3 +-silicates, e.g., piemontites [6]. These high CFSE are due to strong Jahn-Teller stabilization of Mn 3+ in the andalusite matrix and may explain the enlargement of the PT stability field of this structure

Extended x-ray-absorption fine structure (EXAFS) and its application to surface structure

Abstract: A discussion is given of a new class of methods to ascertain the configuration of adsorbates on surfaces. This class uses the extended x‐ray‐absorption fine‐structure (EXAFS) effect, which can be simply related to the local environment about atoms of a particular element. Both ordered and disordered arrays of atoms can be studied and directions perpendicular and parallel to the surface can be distinguished. The results of an EXAFS study of Br2 adsorbed on Grafoil are summarized, illustrating the detailed information obtainable by this technique. It is found that at 0.2 monolayer coverage at room temperature Br2 adsorbs as molecules. One atom of the molecule is fixed to the graphite substrate with the other atom free to flop around randomly. The location of the fixed Br atom is also determined.