摘要:

X‐ray absorption spectroscopy at the C K edge has been performed on carbon nanotubes to demonstrate the capability of the technique to investigate the electronic structure of these systems. We compare and discuss the experimental results achieved on both graphite and nanotubes trying also to determine the relationship between multiple scattering structures and different topology and surface structure of the samples. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536396

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In this paper we present a new method, named MXAN, able to fit the XANES energy range (from the edge to about 200 eV) of experimental X‐ray absorption data to obtain geometrical information. This method is based on the comparison between the experimental spectrum and several theoretical calculations generated by changing the relevant geometrical parameters of the site around the absorbing atom. The theoretical spectra are derived in the framework of full multiple scattering approach. Our procedure is able to recover the right information on the symmetry and atomic distances and the solution is found to be independent on the starting conditions. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536397

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Reliable knowledge of the complex X‐ray form factor [Re(f) and Im(f)] and the photoelectric attenuation coefficient (&sgr;PE) is required for crystallography, medical diagnosis, radiation safety and XAFS studies. Key discrepancies in earlier theoretical work are due to the smoothing of edge structure, the use of non‐relativistic wave functions, and the lack of appropriate convergence of wave functions. These discrepancies lead to significant corrections for most comprehensive (i.e. all‐Z) tabulations. This work has led to a major comprehensive database tabulation [Chantler, C. T. (2000). J. Phys. Chem. Ref. Data, 29, 597–1048] which serves as a sequel and companion to earlier relativistic Dirac‐Fock computations [Chantler, C. T. (1995). J. Phys. Chem. Ref. Data, 24, 71–643]. The paper finds that earlier work needs improvement in the near‐edge region for soft X‐ray energies, and derives new theoretical results of substantially higher accuracy in near‐edge soft X‐ray regions. Fine grids near edges are tabulated demonstrating the current comparison with alternate theory and with available experimental data. The best experimental data and the observed experimental structure as a function of energy are strong indicators of the validity of the current approach. New developments in experimental measurement hold great promise in making critical comparisons with theory in the near future. This work forms the latest component of the FFAST NIST database [http://physics.nist.gov/PhysRefData/FFast02/Text/cover.html]. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536398

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

X‐ray absorption spectroscopy (XANES and EXAFS) has been used to determine the environment of cations (Ni, Zn, Zr, Fe and Mo) in oxide glasses and their redox state. Direct quantitative structural information can be extracted which indicates that cations are often present in unusually low coordination number compared to crystals. Medium range environment can be assessed with second and further neighbors. This yields to define structural models on the connectivity between cation polyhedra and the network structure. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536399

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We report the use of X‐rays absorption technique, detected in total reflection mode, to obtain structural information on new materials. After a brief description of the ReflEXAFS technique, we present the results obtained in the study of two very peculiar solid‐state problem: a) the effect of the Sb as surfactant in the Si/Ge multilayers and b) the understanding of the very early stage of the spinel formation. We show that the use of a Sb film is not enough to completely stop the interdiffusion process and the quality of the interface is quantify in terms of the interdiffusion of Ge in Si. The second case deals with the structural study of the very earl stage of the NiAl2O4spinel formation; this solid‐state reaction requires high temperature and long time of exposition in O2atmosphere. The progresses of the reaction have been followed by several ReflEXAFS measurement, taken after each thermal treatment. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536400

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Here we report the use of EXAFS (Extended X‐ray Absorption Fine Structure) and XANES (X‐ray Absorption Near Edge Structure) spectroscopy method to study the spatial and electronic structure of Ge/Si heterostructures. The pyramid‐like Ge islands deposited on Si(001) substrate using molecular beam epitaxy at 300°C reveal quantum dots (QDs) properties. Measurements of X‐ray absorption fine structure at germaniumKedge (GeKXAFS measurements) have been performed using total electron yield and fluorescent detection mode. It is revealed that the pure Ge nanoclusters are covered by 1,2‐monolayer film with 50&percent; Si atom impurity caused by interface diffusion at 500°C. The Ge QDs are characterized by interatomic Ge‐Ge distances of 2.41Å which is 0.04 Å less than in bulk Ge. The influence of effective thickness of the germanium film, Ge nanocluster sizes and Ge, Si deposition temperature on the QDs microstructure parameters is revealed. The first attempt to extract from X‐ray absorption spectra (XANES spectra) an information about the energy structure of the free states of the quantum dot was made. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536401

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The review of systematic investigations of X‐ray radiation properties of different clusters heated by short‐pulse (35–1100 fs) high‐intensive (1016– 1018W/cm2) Ti:Sa laser radiation is presented. The cluster targets were formed by the adiabatic expansion in vacuum of an Kr or Ar gas jets produced by a pulsed valve with Laval or conical nozzles. The gas pressure is varied from 15 up to 100 bar. High spectrally (&lgr;/&dgr;&lgr;=4000–5000) and spatially (40–80 &mgr;m) resolved X‐Ray spectra near resonance lines (4‐2 transitions) of Ne‐like ions of Kr, H‐ and He‐like ions of Ar have been obtained and detailed spectroscopic analysis was consistent with a theoretical two‐temperature collisional‐radiative model of irradiated atomic clusters incorporating with an effects of highly energetic electrons. The role of laser prepulse for X‐ray intensity emission investigated in details. X‐ray spectra radiation from plasma with electron density more than 2×1022cm−3was observed. Big effect of fast electrons influence on the X‐ray emission of He‐like Ar and Ne‐like Kr spectra was demonstrated. Comparison of data obtained under various experimental conditions clearly showed that for increasing X‐ray output from plasma the most essential to increase size of clusters and has reasonable value of ps prepulse. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536402

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The number of mineral species known to date rapidly approaches 4000, and yet they represent but a small fraction of all the known inorganic and organic compounds. Nevertheless, minerals represent an ideal field of activity for X‐ray absorption spectroscopy (XAS), because the investigation of their crystal‐chemical peculiarities takes an enormous advantage of the property of this method of being atom‐selective, even in the presence of a wide range of competing atoms located in similar structural environments. As a matter of fact, XAS on minerals proved to be a useful probing method as early as for W. Kossel’s pioneer studies of in the 1930’s, just after the fine structures occurring at and near the absorption edge had been first detected. However, XAS did not really become consolidated in mineral studies until the 1980’s, when synchrotron sources became available to users. A concise, but complete review of the historical and recent applications of XAS to minerals and to their analogues synthesized for geological/geophysical purposes i.e., to better understand the mechanisms by which the Earth evolves, is here given. Special reference will be made to transition metals (Ca, Ti, Cr, Mn, Fe, Ni) which absorb in the hard X‐ray spectral region (> 4 KeV) and to the geologically‐significant elements (O, Na, Mg, Al, Si, S and K) which absorb in the soft X‐ray region (500–4000 eV). © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536403

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We describe non‐equilibrium‐state x‐ray absorption spectroscopy focusing on local structure of photo‐excited states trapped at low temperature. For this purpose, a novel Ge 100 pixel array detector with a packing density of 88&percent; was developed. The local structure of photo‐induced phase of Fe(II) spin crossover complex, [Fe(2‐pic)3]Cl2EtOH (2‐pic=2‐aminomethyl pyridine), was investigated at low temperature (T <150 K). The use of pixel array detector and high‐flux synchrotron x‐ray source (multipole wiggler) successfully provided x‐ray absorption spectra with high quality,in‐situ, during the photo‐excitation. It was found thatthe photo‐induced phase under optical pumping at low temperature (T < 50 K) has an octahedral geometry with the elongated Fe‐N distance (2.16 ± 0.01 Å), stabilizing the high spin state (S=2) configuration. No indication of symmetry breaking of FeN6clusters upon LS↔HS spin‐state switching was observed. It was demonstrated that the technique is a promising means to probe the local structure of non‐equilibrium state such as trapped excited states or metastable states. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536404

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Local electronic and structural properties of PrNiO3perovskite were studied by means of X‐ray Absorption Spectroscopy at Ni K and L edges. The EXAFS results at Ni K edge show a structural transition from three different Ni&sngbnd;O bond‐lengths at the insulating phase to two Ni&sngbnd;O bond‐lengths above TMI. These results were interpreted as being due to a transition from a structure with two different Ni sites at the insulating phase to one distorted Ni site at the metallic phase. The Ni L edge spectra show a remarkable difference between the spectra measured at the insulating and metallic phases that indicates a decreasing degree of hybridization between Ni3d and O2p bands from the metallic to the insulating phase. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1536405

出版商:AIP    

年代:1903

数据来源: AIP