离子液体[BMIM]Br与[BMIM][BF4]配比浓度对Br-离子电荷转移的调节

利用X射线吸收精细结构光谱(XAFS)及紫外吸收光谱两种方法, 分析了离子液体1-丁基-3-甲基咪唑溴盐([BMIM]Br)中逐渐掺入1-丁基-3-甲基咪唑四氟硼酸盐([BMIM][BF₄])时, Br^-阴离子与咪唑阳离子之间氢键作用及电荷偏移量的改变. 随着[BMIM][BF₄]加入量增多, Br 元素XAFS近边(XANES)显示吸收峰降低, 吸收边位置向低能端位移0.9 eV; 扩展边(EXAFS)算出径向结构显示Br 与近邻原子间平均配位数降低、平均键长增长; 紫外光谱也有明显蓝移减色效应. 这些结果都表明Br₄^-的掺入改变了Br^-与阳离子间的电荷偏移量, 负电荷更多地转移到Br^-上, 量化计算的数据同样支持该结论.     

X-ray absorption study of the solvation structure of Cu2+ in methanol and dimethyl sulfoxide

The solvation structure of Cu(2+) in methanol (MeOH) and dimethyl sulfoxide (DMSO) has been determined by studying both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) regions of the K-edge absorption spectra. The EXAFS technique has been found to provide a very accurate determination of the next-neighbor coordination distances, but it is inconclusive in the determination of the coordination numbers and polyhedral environment. Conversely, quantitative analysis of the XANES spectra unambiguously shows the presence of an average 5-fold coordination in both the MeOH and DMSO solution, ruling out the usually proposed octahedral Jahn-Teller distorted geometry. The EXAFS and XANES techniques provide coherent values of the Cu-O first-shell distances that are coincident in the two solvents. This investigation shows that the combined analysis of the EXAFS and XANES data allows a reliable determination of the structural properties of electrolyte solutions, which is very difficult to achieve with other experimental techniques.     

Influence of Local Atomic and Electronic Structures of Magnetite on Subtle Effects in HERFD-XANES Spectra

Large iron oxide nanoparticles are studied by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XANES). A theoretical procedure within the finite-difference method framework is tested to calculate Fe K-edge X-ray absorption spectra for the magnetite structure. The influence of structural and non-structural calculation parameters on the subtle effects in the X-ray absorption spectra are considered. Tetrahedral positions of Fe³⁺ ions are shown to provide the main contribution to the spectral feature at 20–21 eV whose intensity grows together with the content of these ions.     

Atomic cluster calculation of the X-ray near-edge absorption of copper

The finite difference method for near-edge structure is used to calculate X-ray absorption near-edge structure (XANES) spectra. We extend the range of calculation for copper above the K-shell threshold and compare the results with recent experimental data in the X-ray absorption fine structure (XAFS) region. Qualitatively the calculation predicts the location of the peaks but fails to accurately describe relative amplitudes.     

Three-dimensional local structure of photoexcited Cu diimine complex refined by quantitative XANES analysis

The structural details of [Cu(dmp) 2] (+) (dmp = 2,9-dimethyl-1,10-phenanthroline) at its metal-to-ligand charge-transfer (MLCT) excited-state in acetonitrile were extracted using quantitative analysis of Cu K-edge X-ray adsorption near edge structure (XANES). The study combines two techniques: fitting experimental XANES spectra with a multidimensional interpolation approximation, and calculating theoretical XANES spectra with molecular potentials beyond the muffin-tin approximation. The results of the study show that the best fit of the experimental XANES data must include a solvent molecule binding to the Cu with a short Cu-N distance of 2.00 A. This confirms that the formation of an exciplex is responsible for the excited-state quenching in coordinating solvents, such as acetonitrile. Moreover, the calculations suggest that the formation of this exciplex state is accompanied by significant rocking distortions of the dmp ligands resulting in a 108 degrees angle between the N(solvent)-Cu bond and the C 2 symmetry axis of the dmp ligand. This combined approach allows us to extract molecular configurations that would otherwise be missed in a conventional qualitative XANES analysis.     

X-ray absorption spectroscopy of nanostructured polyanilines

In this paper, X-ray absorption near edge spectroscopy at the nitrogen K edge (N K XANES) data of polyaniline (PANI) and its derivatives were revisited and expanded. The N K XANES spectra of PANI nanocomposites and PANI nanofibers were also investigated. The analysis of N K XANES spectra were done by the deconvolution of bands and the 1s → π* and 1s → σ* transitions were assigned by a correlation with the N K XANES spectra of smaller organic compounds. The “free” forms of PANI were dominated by bands from 397.7 eV to 399.1 eV attributed to imine and radical cation nitrogen atoms, respectively. Nitrogen bonded to phenazine-like rings can also be seen, mainly for PANI prepared at pH higher than 3.0. The spectra of nanocomposites show sharper bands than the “free” polymers as well as new bands at 398.8 eV and 405–406 eV. These new bands were assigned to phenazine-like rings and azo bonds in the structure of the polymers (polyaniline, polybenzidine, and poly(p-phenylediamine)) within the galleries of the montmorillonite clay. PANI nanofibers doped with HCl or HClO₄ show bands related to phenazine-like rings and/or dication segments of PANI, indicating that these segments are important in the formation of PANI nanofibers.     

Quantitative XAFS/EELS analyses of nitrogen species in titanium oxide photocatalysts

With a view to rational designing of a highly functional visible-light TiO₂ photocatalyst, nitrogen atoms were doped into TiO₂ samples by an ion implantation technique which enables to control the depth and concentration of dopants. Although the absorbance in the visible-light region of the sample increased by the nitrogen doping, photocatalytic activity of the sample was not directly connected with the photo-absorbance. The N K-edge X-ray absorption near edge structure (XANES) spectrum of the photocatalytic active sample (A-cat) showed a characteristic double peak at 398 and 401 eV, and the XANES spectrum of the inactive sample (I-cat) a distinct single peak around 401 eV. These features of the XANES spectra were well reproduced by theoretical simulations based on the model where an O atom in TiO₂ was replaced by N ((N)s) for A-cat, and that of quasi NO₂ molecule ((NO₂)s) for I-cat. Therefore, we have concluded that the nitrogen atom occupying the oxygen site of TiO₂ is photocatalytic active species effective for visible light photocatalysis. In addition, the quantitative XANES/ELNES analysis has revealed that the photo-absorbance ratio of I-cat to A-cat corresponds well to the ratio of total doped nitrogen concentration rather than photocatalytic active nitrogen ((N)s) concentration. This result indicates that not only (N)s but also (NO₂)s also absorb the visible light. Thus, the absorbance in the visible-light region is not necessarily an indication of the visible-light response of a photocatalyst.     

Nitrogen defect levels in InN: XANES study

The local and electronic structure of nitrogen-related defects in thin film of InN (0 0 0 1) has been studied using synchrotron-based X-ray absorption near edge structure (XANES) spectroscopy. Several defect levels within the band gap and the conduction band of InN were clearly resolved in XANES spectra around the nitrogen K-edge. Theoretical analysis of XANES data includes advanced “ab initio” simulations: self-consistent full multiple scattering calculations using muffin-tin approximation, non-muffin-tin finite difference approach to study the influence of non-muffin-tin effects on XANES shape as well as advanced local density approximation scheme for optimization of initial geometry around nitrogen defects. Theoretical analysis of XANES data allows to attribute the level observed at 1.7 eV above the conduction band mimimum to antisite nitrogen and a sharp resonance at 3.2 eV above the conduction band minimum to molecular nitrogen.     

Effect of swift heavy ions in Ni-Al nanocrystalline films studied by X-ray absorption spectroscopy

X-ray absorption spectroscopic measurements have been used to compare the electronic structures of swift heavy ions (100 MeV Si ions) irradiated and pristine Ni-Al nanocrystalline films. Results from X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) spectra at Al K-, and Ni L(2,3)-edges and extended X-ray absorption fine structure (EXAFS) at Ni K-edges are discussed. The observed XRD peaks indicate the improvement of crystalline nature and Al(111) clustering after the swift heavy ion interactions. While the XANES spectra at Ni L(2,3)-edges show decrease in the intensity of white line strength, the Al K-edge shows increase in intensity after irradiation. Above results imply that swift heavy ions induce low Z (i.e., Al) ion mass transport, changes in Al sp-Ni-d hybridization, and charge transfer. EXAFS results show that crystalline nature is improved after swift heavy irradiation which is consistent with XRD results.     

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.     

Ab initio calculation of temperature effects in the optical response of open-shell sodium clusters

Many properties of atomic clusters have been found to be size dependent, e.g., the optical response. There are, however, factors other than size that can also play an important role in determining the properties of nanoscale systems. Temperature, in particular, has been shown to have a strong effect on the optical response of open-shell sodium clusters. We incorporate the temperature effect on the optical absorption spectra by combining pseudopotentials, Langevin molecular dynamics, and time-dependent density functional theory. We have done calculations for several open-shell sodium clusters, Na(4) (+), Na(7) (+), and Na(11) (+), for which experimental data are available for comparison. We find that the positions of the lower energy peaks of the calculated spectra correspond very well to the peaks in the experimental spectra, although the local density approximation tends to overestimate the gap of the smaller clusters by up to 0.2 eV and underestimate the gap of the largest cluster by 0.4 eV. We fit the width of the peaks in the lower-temperature calculations to the corresponding experimental result to obtain the instrumental linewidth. We then use this same width for the high-temperature calculations and find very good agreement with experiment. Finally, we analyze the transitions that contribute to the observed peaks in the absorption spectra and we plot the effective valence charge density for specific transitions for each cluster. We find that for the two smaller clusters the absorption spectra are dominated by transitions from the occupied levels to a few (three for Na(4) (+) and five for Na(7) (+)) empty levels, although the contribution from transitions to other empty levels can still be significant. In contrast, the absorption spectra for Na(11) (+) come from a greater mixture of transitions as evidenced in the analysis as well as in the plot of the effective valence charge density.     

Tuning the magnetic properties of Li(x)CrTi0.25Se2 (0.03 < or = x < or = 0.7) by directed deintercalation of lithium

X-ray diffraction (XRD), in situ energy-dispersive X-ray diffraction (EDXRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and magnetic measurements were applied to investigate the effects of lithium deintercalation on pseudolayered Li(0.70)CrTi(0.25)Se(2). A detailed picture of structural changes during the deintercalation process was obtained by combining the results of EDXRD and EXAFS. Removal of Li from the host-guest complex leads to anisotropic contraction of the unit cell with stronger impact on the c axis, which is the stacking axis of the layers. The EDXRD experiments evidence that the shrinkage of the lattice parameters with decreasing x(Li) in Li(x)CrTi(0.25)Se(2) is nonlinear in the beginning and then becomes linear. Analysis of the EXAFS spectra clearly shows that the Cr/Ti-Se distances are affected in a different manner by Li removal. The Cr-Se bond lengths decrease, whereas the Ti-Se bonds lengthen when the Li content is reduced, which is consistent with XRD data. Magnetic measurements reveal a change from predominantly antiferromagnetic exchange (theta(p) = -300 K) interactions for the pristine material to ferromagnetic exchange interactions (theta = 25 K) for the fully intercalated material. Thus, the magnetic properties can be altered under ambient conditions by directed adjustment of the dominant magnetic exchange. The unusual magnetic behavior can be explained on the basis of the variation of the metal-metal distances and the Cr-Se-Cr angles with x, which were determined by Rietveld refinements. Owing to competing ferromagnetic and antiferromagnetic exchange interactions and disorder, the magnetic ground state of the intercalated materials is characterized by spin-glass or spin-glass-like behavior.     

XAS study on copper red in ancient glass beads from Thailand

Glass has been used in ornaments and decorations in Thailand for thousands of years, being discovered in several archeological sites and preserved in museums throughout the country. To date only a few of them have been examined by conventional methods for their compositions and colorations. In this work we report for the first time an advanced structural analysis of Thai ancient glass beads using synchrotron X-ray absorption spectroscopy (XAS) and energy-dispersive X-ray (EDX) spectrometry. Four samples of ancient glass beads were selected from four different archeological sites in three southern provinces (Ranong, Krabi and Pang-nga) of Thailand. Archaeological dating indicated that they were made more than 1,300 years ago. A historically known method for obtaining a red color is to add compounds containing transition elements such as gold, copper, and chromium. For our samples, EDX spectrometry data revealed existing fractions of iron, copper, zinc, and chromium in ascending order. Thus, copper was selectively studied by XAS as being potentially responsible for the red color in the glass beads. K-shell X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of copper were recorded in fluorescence mode using an advanced 13-element germanium detector. Comparisons with XANES spectra of reference compounds identified two major forms of copper, monovalent copper and a metallic cluster, dispersed in the glass matrix. The cluster dimension was approximated on the basis of structural modeling and a theoretical XANES calculation. As a complement, EXAFS spectra were analyzed to determine the first-shell coordination around copper. XAS was proven to be an outstanding, advanced technique that can be applied to study nondestructively archaeological objects to understand their characteristics and how they were produced in ancient times.     

Small copper nanoclusters: X-ray absorption analysis

X-ray absorption near edge structure (XANES) spectra at the cooper L_2,3-edge of small cooper nanoclusters has been studied. Theoretical interpretation of XANES spectra measured for small copper nanoclusters have been performed. Theoretical analysis is based on a self-consistent full multiple scattering theory of X-ray absorption as well as on non-muffin-tin finite difference method (FDM). This approach allows choosing the best model for local geometry of small cooper nanoclusters.     

X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers—potential and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis

X-ray absorption spectroscopy (XAS) has become a prominent tool for the element-specific analysis of transition metals at the catalytic center of metalloenzymes. In the present study the information content of X-ray spectra with respect to the nuclear geometry and, in particular, to the electronic structure of the protein-bound metal ions is explored using the manganese complex of photosystem II (PSIII) as a model system. The EXAFS range carries direct information on the number and distances of ligands as well as on the chemical type of the ligand donor function. For first-sphere ligands and second-sphere metals (in multinuclear complexes), the determination of precise distances is mostly straightforward, whereas the determination of coordination numbers clearly requires more effort. The EXAFS section starts with an exemplifying discussion of a PSII spectrum data set with focus on the coordination number problem. Subsequently, the method of linear dichroism EXAFS spectroscopy is introduced and it is shown how the EXAFS data leads to an atomic resolution model for the tetra-manganese complex of PSII. In the XANES section the following aspects are considered: (1) Alternative approaches are evaluated for determination of the metal-oxidation state by comparison with a series of model compounds. (2) The interpretation of XANES spectra in terms of molecular orbitals (MOs) is approached by comparative multiple-scattering calculations and MO calculations. (3) The underlying reasons for the oxidation-state dependence of the XANES spectra are explored. Furthermore, the potential of modern XANES theory is demonstrated by presenting first simulations of the dichroism in the XANES spectra of the PSII manganese complex.     

基于SRXRF和XANES研究Pb对玉米种子萌芽的影响及其分布和形态特征

以受不同浓度Pb胁迫下的玉米种子萌芽试验为基础,结合SRXRF研究Pb对玉米种子萌芽的影响和Pb在玉米种子的微区分布特征,利用X射线吸收近边结构技术(XANES)对萌芽玉米种子不同部位中Pb的化学形态进行分析,藉以了解种子对Pb的吸收和转化规律。结果表明,玉米种子的发芽率、根芽伸长量随培养溶液中Pb(NO3)2浓度增加而降低。种子发芽率、平均芽长、平均根长与暴露Pb浓度间的方差分析P-value分别为2.0×10!3,1.4×10!4和2.39×10!8,均小于0.01,为极显著差异,说明Pb胁迫对种子萌芽和根芽生长造成了极显著影响,且对根长抑制作用更大。SRXRF分析结果表明,种皮和胚结构是玉米种子富集Pb的主要部位,Pb富集在胚结构中将抑制种子萌芽。各种子样品的Pb-LⅢ(13035 eV)边XANES谱图相同,为同一种Pb形态,拟合结果显示其含74.3%的氯化磷酸铅和24.2%硬脂酸铅,说明Pb主要以氯化磷酸铅的形式沉积于玉米体内,并有少部分以与羧基结合的有机铅形式存在。     

X-ray absorption spectroscopy of hemes and hemeproteins in solution: multiple scattering analysis

A full quantitative analysis of Fe K-edge X-ray absorption spectra has been performed for hemes in two porphynato complexes, that is, iron(III) tetraphenylporphyrin chloride (Fe(III)TPPCl) and iron(III) tetraphenylporphyrin bis(imidazole) (Fe(III)TPP(Imid)2), in two protein complexes whose X-ray structure is known at atomic resolution (1.0 A), that is, ferrous deoxy-myoglobin (Fe(II)Mb) and ferric aquo-myoglobin (Fe(III)MbH2O), and in ferric cyano-myoglobin (Fe(III)MbCN), whose X-ray structure is known at lower resolution (1.4 A). The analysis has been performed via the multiple scattering approach, starting from a muffin tin approximation of the molecular potential. The Fe-heme structure has been obtained by analyzing independently the Extended X-ray Absorption Fine Structure (EXAFS) region and the X-ray Absorption Near Edge Structure (XANES) region. The EXAFS structural results are in full agreement with the crystallographic values of the models, with an accuracy of +/- 0.02 A for Fe-ligand distances, and +/-6 degrees for angular parameters. All the XANES features above the theoretical zero energy (in the lower rising edge) are well accounted for by single-channel calculations, for both Fe(II) and Fe(III) hemes, and the Fe-N p distance is determined with the same accuracy as EXAFS. XANES evaluations of Fe-5th and Fe-6th ligand distances are determined with 0.04-0.07 A accuracy; a small discrepancy with EXAFS (0.01 to 0.05 A beyond the statistical error), is found for protein compounds. Concerns from statistical correlation among parameters and multiple minima in the parameter space are discussed. As expected, the XANES accuracy is slightly lower than what was found for polarized XANES on Fe(III)MbCN single crystal (0.03-0.04 A), and states the actual state-of-the-art of XANES analysis when used to extract heme-normal parameters in a solution spectrum dominated by heme-plane scattering.