Multiple-scattering Approach Research Articles

Multiple-scattering approach to near-edge X-ray absorption fine structure of CO/NiO(100) and NO/NiO(100)

The carbon and nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) of CO/NiO(100) and NO/NiO(100) were calculated by the multiple-scattering cluster (MSC) method. The calculation shows that the shape resonance in C 1s NEXAFS spectra is attributed to the photoelectron scattered between the carbon atom and the substrate, and the weak features near the π-resonance are caused by the shake-up effect. R-factor calculation demonstrates for the first time that the CO molecule is adsorbed at the asymmetric bridge site of the NiO(100) surface; the adsorption height of the carbon atom in the CO molecule is equal to 0.31±0.01 nm. Our MSC calculation supports former ab initio studies and the experimental results, i.e., CO/NiO(100) is a physisorption system, while NO/NiO(100) is a weak chemisorption system.

X-ray Absorption Spectroscopic Studies of the Copper(I) Complexes of Crown Ether Appended Bis{(2-pyridyl)ethyl}amines and Their Dioxygen Adducts.

An X-ray absorption spectroscopic study of the Cu complexes of the bis{(2-pyridyl)ethyl}-appended monoaza crown ether 1, diaza crown ether 2, and diphenylglycoluril diaza basket 3 is reported. Following detailed analysis of the spectra of the crystallographically characterized model compound tetrapyridyl Cu(II) bis(nitrato pyridine) (4), the contributions of the ring atoms of the coordinating pyridine to the EXAFS were simulated using a multiple-scattering approach and the final parameters obtained by restrained refinement. Oxygenation of the Cu(I) complexes resulted in a large increase of the intensity of the major peak in the phase-corrected Fourier transform. This was interpreted as evidence for a &mgr;-eta(2):eta(2)-peroxo coordination mode of the oxygen between the copper ions, which had changed valence from Cu(I) to Cu(II) as judged from the edge position. This oxygen binding mode is reminiscent of that of hemocyanin, but the Cu-Cu distances are significantly shorter in the model than in the enzyme and vary with solvent. The EXAFS of the oxygenated complexes was simulated in a new approach in which, besides the parameters of the pyridine unit, those of an additional multiple scattering unit describing the geometry of the Cu-O(2)-Cu moiety were also refined.

Full Multiple Scattering Approach to X-Ray Absorption near Edge Structure Spectra of Bromonaphthalene in Supercritical Xenon

Detailed X-ray absorption near edge structure (XANES) analyses are carried out in order to study the local structure around a Br atom of bromonaphthalene dissolved in supercritical fluid Xe by usage of full multiple scattering calculations. We find that about six Xe atoms surround the Br atom at the distance of about 5 Å, which is larger than the sum of the van der Waals radii (4.15 Å). Other important information is the relative thermal fluctuation (Debye-Waller factor) between Br and Xe, which is quite large, ∼0.8 Å. This large thermal fluctuation and clustering are essential characteristic features of supercritical systems.

Multiple-scattering theory for three-dimensional periodic acoustic composites

We present results for acoustic wave propagation in periodic composites consisting of solid spheres in a fluid host. We show that for solid scatterers in fluid host material combinations the extensively used plane-wave method is inadequate to produce accurate results and a new approach is required. Our band-structure results are obtained by using a multiple-scattering approach based on an extension of the well-known Korringa-KohnRostoker method. @S0163-1829~99!04841-9#

X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach

The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO2 and CeO2 are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific atomic arrangement and electronic structure. The structures at the edge originate from oxygen 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighbouring shells of the central oxygen atoms.

Electronic density of empty states of Ge/Si(111) epitaxial layers: Theory and experiment

The structural and morphological properties of thin Ge layers deposited on $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ substrate kept at room temperature and subsequently annealed at several temperatures have been investigated using near-edge x-ray absorption spectroscopy (XAS) at the Ge L edges. Experimental data have been compared with spectra calculated by a multiple-scattering approach in order to obtain the best-fitting interface model. Our results indicate an amorphous growth at room temperature, for coverages ranging from 3 to 14 ML. Two different behaviors are observed after annealing for Ge coverages below and above 3 ML, the so-called critical thickness. For very thin layers, XAS suggests the formation of a continuous crystalline structure driven by strong intermixing processes. Ge layers thicker than the critical thickness have been found to form three-dimensional islands characterized by a lattice-parameter relaxation and dispersed on a wetting layer.

Relativistic description of valence energy losses in the interaction of fast electrons with clusters of dielectrics: Multiple-scattering approach

A fully relativistic description of valence energy losses suffered by fast electrons passing near finite clusters of arbitrarily disposed dielectric objects is presented using an accurate technique suited to solve Maxwell{close_quote}s equations. The method is based upon an expansion of the electromagnetic field in terms of multipoles around each of the objects of the cluster. Multiple elastic scattering of those multipole expansions is then performed until convergence is achieved. The energy loss, obtained from the induced electric field acting back on the electron, is computed in a time proportional to the square of the number of objects in the cluster, N{sup 2}. Numerical examples are presented for various clusters formed by N=1{endash}198 homogeneous spheres made of SiO{sub 2} and Al, and also for clusters of Si spheres coated with SiO{sub 2}. Both relativistic effects and the interaction between the constituents of the cluster are shown to be of primary importance in the understanding of the position and magnitude of the features exhibited by the calculated electron-energy-loss spectra. {copyright} {ital 1999} {ital The American Physical Society}

Diffuse radiation intensity propagating through a particulate slab

A multiple-scattering approach is used to solve the radiative transfer equation in order to describe the angular and optical depth dependence of diffuse intensity propagating through an unsupported particulate slab perpendicularly illuminated with unpolarized electromagnetic radiation. Boundary reflections are taken into account for the unscattered radiation, as is each of the successive reflection and scattering orders, which are characterized by corresponding average path-length parameters and forward-scattering ratios. The optical depth dependence of the reflection- and scattering-order coefficients is displayed for given values of the particle size parameter, particle volume fraction, relative refractive index, and optical thickness of the slab. Diffuse intensity patterns at a given optical depth, and the angular dependence of the reflected and transmitted intensities, are also considered.

Multiple-scattering theoretical approach to magnetic dichroism and spin polarization in angle-resolved core-level photoemission

Magnetic dichroism in spin- and angle-resolved core-level photoemission is investigated within the relativistic one-step model of photoemission based on multiple-scattering theory. Photoelectron scattering in emission from $3p$ levels of low-index Fe and Ni surfaces is found to affect intensities, dichroism, and photoelectron spin polarizations considerably, in particular in off-normal emission. In general, the calculated spectra agree well with their experimental counterparts in spectral shape, size of the dichroic signal, and spin polarization. Magnetic circular dichroism in a chiral setup is analyzed theoretically using appropriately defined asymmetries. Further, core-level energy positions and photoemission are compared with those obtained within an atomic model.

Multiple-scattering calculations of electron-energy-loss near-edge structures of existing and predicted phases in the ternary system B-C-N

The electron-energy-loss near-edge structures (ELNES) of the B $K,$ C $K,$ and N $K$ ionization edges of different phases in the boron-carbon-nitrogen (B-C-N) system are calculated using a multiple scattering (MS) approach. A comparison of calculated and experimental spectra for cubic diamond, graphite, cubic $(c$-BN) and hexagonal $(h$-BN) boron nitride is made to judge the reliability of the MS approach. The calculations are found to reproduce the main features of the experimental ELNES. However, in the quantitative comparison the peak positions deviate up to 3 eV. The multiple-scattering approach is applied to predict the ELNES of three hypothetical phases, namely, ${\mathrm{BC}}_{2}\mathrm{N},$ ${\mathrm{BC}}_{3},$ and $\ensuremath{\beta}\ensuremath{-}{\mathrm{C}}_{3}{\mathrm{N}}_{4},$ for which no accepted experimental reference spectra exist. Our results demonstrate the usefulness of MS calculations to predict the ELNES of novel phases as a tool for future experimental characterization.

Real-space multiple-scattering approach to XANES.

Real-space multiple-scattering approach to XANES.

First-principles exchange interactions in Fe, Ni, and Co

Using a multiple-scattering approach and the local-density and rigid-spin approximations, the exchange interactions are calculated for Fe, Ni, and Co, and the spin-wave spectrum obtained. The interactions contributing to the spin-wave spectrum are found to be rather long-ranged. The exchange parameters are calculated as a function of band filling, and predict transition to antiferromagnetic behavior for electron-deficient alloys of bcc Fe and fcc Co.

Noncollinear magnetism in substitutionally disordered face-centered-cubic FeMn

We use first principles electronic structure techniques to study the magnetic structure of γ-FeMn using the Korringa–Kohn–Rostocker multiple-scattering approach in conjunction with an extension of the single site coherent potential approximation to noncollinear magnetic structures. Our results show that the noncollinear 3Q and 2Q structures are both stable solutions with the former being slightly lower in energy. The collinear solutions could only be converged in a traditional spin-polarized calculation and are unstable in a noncollinear treatment.

Interaction of Radiation and Fast Electrons with Clusters of Dielectrics: A Multiple Scattering Approach

A fast, accurate, and general technique for solving Maxwell's equations in arbitrarily disposed clusters of dielectric objects is presented, based upon multiple scattering of electromagnetic multipole fields. Examples of application to the simulation of electron energy loss, radiation emission induced by fast electrons, and light scattering are offered. Large rates of Smith-Purcell radiation are predicted in the interaction of fast electrons with strings of Al and ${\mathrm{SiO}}_{2}$ spheres, suggesting its possible application in tunable soft UV light generation. Mutual electromagnetic interaction among objects in the different clusters under consideration is shown to be of primary importance.

Multiple-scattering approach to finite-sized photonic band-gap materials

Multiple-scattering theory is used to study the transmission, scattering, and radiation properties of finite-sized photonic band-gap materials in two dimensions. By adopting a source with a finite beam width to simulate realistic experimental conditions, we are able to obtain transmission results that are in excellent quantitative agreements with the measured data. Also, we find that the apparent gaps seen by an antenna can be much larger than the full gaps of the system. Inside an apparent gap, analytic results have been obtained for the radiation patterns and the total radiation power.

Real-space multiple-scattering calculation and interpretation of x-ray-absorption near-edge structure

A self-consistent real-space multiple-scattering (RSMS) approach for calculations of x-ray-absorption near-edge structure (XANES) is presented and implemented in an ab initio code applicable to arbitrary aperiodic or periodic systems. This approach yields a quantitative interpretation of XANES based on simultaneous, self-consistent-field (SCF) calculations of local electronic structure and x-ray absorption spectra, which include full multiple scattering from atoms within a small cluster and the contributions of high-order MS from scatterers outside that cluster. In addition, the code includes a SCF estimate of the Fermi energy and an account of orbital occupancy and charge transfer. We also present a qualitative, scattering-theoretic interpretation of XANES. Sample applications are presented for cubic BN, ${\mathrm{UF}}_{6},$ Pu hydrates, and distorted ${\mathrm{PbTiO}}_{3}.$ Limitations and various extensions are also discussed.

Exchange mechanisms at the Ge/Si(001) interface from a multiple-scattering analysis of the GeL3absorption edge

Intermixing in Ge/Si(001) systems has been recently observed and has emerged as a crucial and controversial topic in the study of Ge first stages of interface formation. In this paper we investigated the structure of Ge overlayers epitaxially grown on Si(001) substrates by a refined approach to x-ray absorption spectroscopy (XAS) at the Ge ${L}_{3}$ edge. XAS experimental spectra, recorded in situ using plane polarized synchrotron radiation, have been directly compared to several Ge ${L}_{3}$ near-edge spectral features calculated for different Ge/Si(001) growth models making use of a multiple-scattering approach. This analysis clearly excludes a layer-by-layer Ge laminar growth and confirms the occurrence of intermixing processes even in the case of Ge room temperature deposition. Moreover, information on the interface structure has been evidenced and the occurrence of a preferential double-layer ordering mechanism along the $〈111〉$ crystallographic directions has been singled out.

A multiple-scattering approach to X-ray photoelectron diffraction

We present an alternative method for multiple scattering calculations of X-ray photoelectron diffraction from periodic surfaces. The technique uses a time-reversed RHEED wave function as the final state in a one-step model of the photoemission process. We apply the method to the Ni(001)c(2×2)S system and find an excellent agreement with the experimental data for vertical height of the S overlayer of 1.35±0.05 Å above the top Ni layer. Our calculations display a much better agreement with the experimental data than has been previously reported.

Dynamics of plume propagation and splitting during pulsed-laser ablation of Si in He and Ar

A modeling approach for calculating the expansion of a laser-generated plasma into a background gas has been developed. Although relatively simple in structure, the model gives excellent fits to various experimental data for Si in background gases of He and Ar, including the previously unexplained ``splitting'' of the ablated plume. The model is based on a combination of multiple-scattering and hydrodynamic approaches. It allows the plume to be broken up into components, or scattering orders, whose particles undergo 0, 1, 2,\dots{} collisions with the background. Particles can only be transferred from one order to the next higher order by collisions. The densities in the individual orders propagate according to the usual conservation equations to give the overall plume expansion. When Ar is the background gas, there is a non-negligible probability that Si plume atoms will reach the detector without undergoing any collisions. This gives rise to a flux component that is undisplaced from that obtained when no background gas is present in addition to the delayed peak from the scattered flux. In Ar only a few orders are necessary for convergence. The behavior in the light gas He is more complex because of the relatively small effect of any one-scattering event and the calculations must be carried out in some cases to as high as the 12th scattering order to find agreement with the experiments.

Multiple Scattering Approach to Pd L3-Edge X-Ray Absorption near Edge Structure Spectra for Small Pd Clusters with Hydrogen Adsorption and Absorption

Recent studies on Pd L3-edge X-ray absorption near edge structure (XANES) spectra for pure small Pd clusters show that the spectra are strongly influenced by hydrogen adsorption and absorption. In this paper, the full multiple scattering method is applied to study the difference in measured spectra. The calculated XANES spectra are not so sensitive to the detailed positions and the electronic structures of the interacting hydrogen atoms, however they are sensitive to the induced structural and electronic state change in the host small Pd clusters. The size effect on the peak at ∼7 eV from the edge is well explained by the induced lattice expansion after hydrogen absorption.