Extended x-ray absorption fine structure studies of oriented single crystals

Linearly polarized synchrotron radiation is used to study the orientation dependence of the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of single crystal zinc. The spectrum some 5–1200 eV above the K photoabsorption threshold is found to be markedly anisotropic, as would be expected from the established formalism generalized to crystals of lower than cubic symmetry. Zinc thus provides a prototype for molecular crystals of low symmetry where one wishes to study particular bond directions.     

Extended X-ray absorption fine structure measurements of laser-shocked V and Ti and crystal phase transformation in Ti

Extended x-ray absorption fine structure (EXAFS), using a laser-imploded target as a source, can yield the properties of laser-shocked metals on a nanosecond time scale. EXAFS measurements of vanadium shocked to approximately 0.4 Mbar yield the compression and temperature in good agreement with hydrodynamic simulations and shock-speed measurements. In laser-shocked titanium at the same pressure, the EXAPS modulation damping is much higher than is warranted by the predicted temperature increase. This is shown to be due to the alpha-Ti to omega-Ti crystal phase transformation, known to occur below approximately 0.1 Mbar for slower shock waves.     

Extended X-Ray absorption fine structure from hydrogen atoms in water

We report the first quantitative measurement of extended x-ray absorption fine structure (EXAFS) from hydrogen atoms. A single oscillation is observed from gaseous water consistent with the location of the covalently bonded hydrogen in H 2O. The experimental phase and amplitude of the oscillation are in excellent agreement with curved wave multiple scattering calculations for isolated water molecules. With this determination of the O-H scattering phase shift we have quantified the covalent hydrogen bond distance (0.95+/-0.03 A) in liquid water, thus demonstrating that hydrogen EXAFS can become a valuable complement to existing structural methods in chemistry and biology.     

Excited state absorption in thulium doped YVO4 crystals

Excited state absorption (ESA) of Tm³⁺ ions in YVO₄ crystal was measured using a pump and probe technique. The measurements have been performed in the wide spectral range in both the near infrared and visible region between 6000 cm^-1 (NIR) and 24000 cm^-1 (VIS). ESA absorption cross section spectra have been calculated for transitions from the ³ F ₄ and ³ H ₄ states of Tm³⁺ and compared to those evaluated experimentally. The excited state absorption in YVO₄:Tm³⁺ should not influence the laser operation related to the ³F₄→³H₆ transition around 1900 nm, but it will be a significant loss factor for a potential laser action associated with the ³H₄→³F₄ transition around 1.48 μm. PACS 42.55.Xi; 42.62.Fi     

Extended X-ray absorption fine structure (EXAFS) study of CaSO4:Dy phosphors

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO₄:Dy phosphors at the Dy L₃ edge with synchrotron radiation. The data have been analysed to find out the Dy-S and Dy-O bond lengths in the neighborhood of the Dy atoms. Measurements have been carried out over several samples thermally annealed for different cycles at 400 °C in air for 1 h and the change in bond lengths in samples with increasing number of annealing cycles have been studied by analyzing the EXAFS data.     

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) study of melt-spun Fe85Ga15 ribbons

The local structure in melt-spun Fe₈₅Ga₁₅ ribbons with a width ∼3 mm and thickness ∼60 μm produced in argon atmosphere was studied by analyzing EXAFS and XANES data. The following results were obtained: Ga–Ga bonds were not detected excluding the tendency to form clusters of Ga atoms; Ga substitutes Fe creating a local strain of about +1% on the first shell Fe–Ga bond, whereas on the second Fe–Ga shell strain quickly relaxes down to +0.3%; XANES spectra are compatible with a random substitution of Fe atoms by Ga atoms in the A2 structure. From the AFM investigation, we observed that at the surface (free side) of the ribbon the particles are elongated along the ribbon (∼2 μm×∼5 μm) and each particle is formed by small grains of average size of 200 nm.     

Near-edge X-ray absorption fine-structure studies of GaN under low-energy nitrogen ion bombardment

The electronic structure of p-type GaN layers exposed to low-energy nitrogen ion bombardment was studied by near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. It was found that ion bombardment lead to the creation of states lying below the nitrogen absorption edge which posses p-symmetry. These states are attributed to nitrogen interstitials with different local topologies created during ion bombardment. Furthermore, the NEXAFS spectra also shows the development of a strong -resonance above the absorption edge with increasing incident nitrogen ion energy. This peak is attributed to the formation of molecular nitrogen at interstitial positions, arising from a build up of nitrogen ions on these sites.     

Finite difference method calculations of X-ray absorption fine structure for copper

The finite difference method is extended to calculate X-ray absorption fine structure (XAFS) for solid state copper. These extensions include the incorporation of a Monte Carlo frozen phonon technique to simulate the effect of thermal vibrations under a correlated Debye–Waller model, and the inclusion of broadening effects from inelastic processes. Spectra are obtained over an energy range in excess of 300 eV above the K absorption edge—more than twice the greatest energy range previously reported for a solid state calculation using this method. We find this method is highly sensitive to values of the photoelectron inelastic mean free path, allowing us to probe the accuracy of current models of this parameter, particularly at low energies. We therefore find that experimental data for the photoelectron inelastic mean free path can be obtained by this method. Our results compare favourably with high precision measurements of the X-ray mass attenuation coefficient for copper, reaching agreement to within 3%, and improving previous results using the finite difference method by an order of magnitude.     

Many-particle-effects in the theory of the extended X-ray absorption fine structure

The Lee-Beni-procedure for the calculation of the extended X-ray absorption fine structure (EXAFS) is extended so as to include the effects of the electronic charge density outside the localized muffin-tin potentials. In our scheme EXAFS is caused by back-scattering of an elementary excitation of a homogeneous electron gas by localized energy dependent many-particle muffin-tin potentials. The difference between the two schemes is negligible at large k's, as expected from physical grounds. However, at small and intermediate k-values the difference is quite large. The effect of the outer electrons as compared to the Lee-Beni-model is twofold. First, they renormalize the scattered electron in the usual way. Second, they are missing within the scattering muffin-tins. Hence, we avoid to count some of the electrons twice. Results are presented for Cu as an example.     

Size-selective extended X-ray absorption fine structure spectroscopy of free selenium clusters

In a recent paper [M. Yao et al., J. Synchrotron Radiat. 8, 542 (2001)]], we proposed a new method for the size-selective EXAFS (extended x-ray absorption fine structure) of neutral-free clusters, in which not only the x-ray absorption process but also the deexcitation processes are utilized as the structural information. In order to verify this method experimentally, we have developed the synchronous measurements of EXAFS and photoelectron photoion coincidence and carried them out for a Se cluster beam by utilizing the third-generation intense x-ray source. The EXAFS spectra for Se small clusters have been obtained and compared critically with theoretical predictions.     

Extended x-ray absorption fine structure study of local structure and atomic correlations of tellurium nanoparticles

Extended x-ray absorption fine structure measurements were performed on trigonal tellurium (t-Te) and tellurium nanoparticles in very thin films. Twofold coordinated chains, much like those in t-Te, exist even in the nanoparticles, but covalent bonds or intrachain interactions are stronger than those of t-Te as shown by a shorter bond length and higher Einstein temperature. The interchain coordination number of the nanoparticles is about half that of t-Te, suggesting reduction of interchain interactions.     

X-ray absorption studies of the structure of nanocrystalline oxides

X-ray absorption spectroscopy is a prime tool in probing the local structure in condensed matter. This contribution describes some of the applications of this technique to probe the microstructure of nanocrystalline oxides. By the use of specific examples four areas will be covered, namely the effect of grain size and preparation route on microstructure, the locating of dopant sites and the chemical nature of metal/oxide catalysts. The materials involved are tin oxide, lithium niobate, titanium dioxide and copper/ceria.     

Extended x-ray absorption fine structure investigations of Nb3Ge0.26Al0.74

We have performed Extended X-ray Absorption Fine Structure (EXAFS) studies of the high-T_c ternary superconducting alloy Nb₃Ge_0.26Al_0.74 above the Ge K-edge. The EXAFS indicates that the local environment surrounding the Ge atoms is characterized by a high degree of structural order, similar to that found in previous investigations of high-T_c Nb₃Ge thin films. We also present direct “microscopic” structural evidence that this material is a solid solution of Nb₃Ge and Nb₃Al. No evidence of Nb₃Ge microcrystals is observed.     

Excited state absorption and energy transfer losses in thulium doped fluoroindogallate glass

Thulium doped fluoroindogallate glass was characterized by means of excited state absorption experiment in the 0.95 to 1.55 μm spectral range. The three bands corresponding to the electronic transitions ³ F ₄→³ F ₂ (at 1.05 μm), ³ F ₄→³ F ₃ (at 1.125 μm), and ³ F ₄→³ H ₄ (at 1.45 μm) were observed. The energy transfer microscopic parameters for the reverse cross relaxation process ³ F ₄, ³ F ₄→³ H ₆, ³ H ₄ were calculated for different multipolar interaction mechanisms using the Kushida model, and it was verified that the probability of this process is 100 times lower than that of the direct ³ H ₄, ³ H ₆→³ F ₄, ³ F ₄ cross relaxation, responsible for the 1.8 μm emission pumping. PACS 78.20.-e; 78.55.Qr; 42.70.Hj; 42.55.Wd     

二己二硫醚多层膜局域结构的近边x射线吸收精细结构研究

利用多重散射团簇(MSC)方法计算了二己二硫醚［CH₃(CH₂)₅S］₂单分子和多分子硫原子近边x射线吸收精细结构(NEXAFS)谱，给出了二己二硫醚多层膜的局域结构模型. MSC研究显示多层膜中二己二硫醚分子作平行有序排列，彼此相距0.47nm，其横截面呈规则的正方形. 利用离散变分Xα方法计算了二己二硫醚单分子和多分子的电子结构，验证了MSC的计算结果；并阐明了NEXAFS谱中各峰的物理起源. 对多层膜中分子之间的相互作用进行了讨论，发现多层膜的局域结构有分子自组装的特性. 关键词： 3(CH₂)₅S］₂多层膜')" href="#">二己二硫醚［CH₃(CH₂)₅S］₂多层膜 近边x射线吸收精细结构 多重散射团簇方法 离散变分Xα方法     

Effects of proton irradiation on structure of NdFeB permanent magnets studied by X-ray diffraction and X-ray absorption fine structure

The effects of proton irradiation on the structure of NdFeB permanent magnet were investigated by X-ray diffraction and X-ray absorption fine structure (XAFS). The results reveal that proton irradiation has no effect on the long-range structure, but significantly affects the atomic local structure of the NdFeB magnet. The alignment degree of the magnet decreases and the internal stress of the lattice increases after proton irradiation. XAFS results show that the coordination number of Fe-Nd in the first neighboring coordination shell of the Fe atoms decreases and the disorder degree increases.     

Thermal stability of GaN epilayers with different degrees of mosaic structure order

Thermal stability of GaN epilayers grown through metal-organic compound vapor deposition on a (0001) sapphire substrate was studied using atomic-force spectroscopy. Samples differing in quality were thermally annealed in ultrahigh vacuum in the range 700–950°C. Mosaic spread in the epitaxial layers is shown to strongly affect their thermal stability. Epilayers with a well-ordered mosaic structure exhibit surface degradation at a temperature of ～950°C. The surface morphology of layers with a large mosaic spread starts to change at lower temperatures, ～780°C.     

神光III原型上扩展X射线吸收精细结构谱实验研究

介绍了在大型激光装置上进行 扩展X射线吸收精细结构(EXAFS)谱压缩物态参数测量的基本原理,以及为获得高质量EXAFS谱在神光III原型装置上进行的实验研究。实验采用玻璃靶球、CH靶丸和金球作为EXAFS谱测量的背光源,通过多发次叠加、光子数累积的方法获得了信噪比良好的金属Ti在常温常压下的EXAFS谱,数据处理结果表明,实验测得的金属Ti EXAFS谱拟合结果与同步辐射实验拟合结果相吻合,表明实验设计的正确性与可靠性。对实验结果的分析表明,影响EXAFS谱质量的因素主要是光子计数、测量系统谱分辨率、噪声以及实验器件上的瑕疵。