Different Kinds Of Atoms Research Articles

Accumulative Transfer of Transverse Magnetic Moment between Spin-Locked Rb and Cs Atoms

We propose an efficient method of collisional transfer of transverse magnetic moment between different kinds of atoms using the spin-locking technique. Demonstrating experiments with a rotating or an oscillating rf field are carried out on a 85 Rb– 133 Cs vapor, and the transfer of a large amount of the transverse magnetic moment is observed when the amplitude H 1 of the rf field is comparable to the static magnetic field strength H 0 . The H 1 -dependence of the transfer, in the case of the oscillating rf field, is not simple, because of the Bloch-Siegert shift and associated resonance effects. The transfer of the transverse magnetic moment under the strong rf field is numerically analyzed with the Bloch equations for 85 Rb and 133 Cs coupled through spin-exchange collisions.

Binary icosahedral clusters: Atom and electron counting rules

The first part of this paper deals with the enumeration of all possible stereoisomers (or polytypes) of a binary (two-colored) icosahedron (either noncentered or centered) via Polya’s theorem. The numbers of achiral and chiral structures for each combination of the two types of atoms (or colors) A and B are determined. In cluster chemistry, A and B may represent two different kinds of atoms (e.g., main-group elements vs transition metals) in a heteronuclear cluster, two different ligand environments in a homonuclear cluster, or atoms and holes (vacant sites) in an icosahedral cluster with missing vertices. The second part deals with the electronic requirements of binary icosahedral clusters where A and B may represent main-group elements and transition metals, or, atoms and holes. Significantly different electron counts are observed for centered vs noncentered icosahedral clusters, depending upon the nature of the interstitial atom. Thus, icosahedral clusters can be classified into four broad categories according to their electron counts: Type I: the majority are those with 13 electron pairs as represented by noncentered icosahedral clusters, or those centered with main-group elements or early transition metals as the interstitial atom; Type II: those with more than 13 electron pairs as exemplified by centered icosahedral clusters with late transition metals as the interstitial atom; and Types III and IV: those with less than 13 electron pairs as observed for centered Au icosahedral clusters. The atom and electron counting rules developed are rationalized in terms of molecular orbital theory. The utilities of the atom and electron counting rules in systematizing structural and spectroscopic data of icosahedral clusters (in both solid-state and gas-phase) as well as in rationalizing the variations of the electron counts in different icosahedral cluster systems are discussed.

A molecular dynamics study of solvent behavior around a protein.

The solvent structure and behavior around a protein were examined by analyzing a trajectory of molecular dynamics simulation of the trp-holorepressor in a periodic box of water. The calculated self diffusion coefficient indicated that the solvent within 10 A of the protein had lower mobility. Examination of the solvent diffusion around different atoms of different kinds of residues showed no general tendency. This fact suggested that the solvent mobility is not influenced significantly by the kind of the atom or residue they solvated. Distribution analysis around the protein revealed two peaks of water oxygen: a sharp one at 2.8 A around polar and charged atoms and a broad one at approximately 3.4 A around apolar atoms. The former was stabilized by water-protein hydrogen bonds, and the latter was stabilized by water-water hydrogen bonds, suggesting the existence of a hydrophobic shell. An analysis of protein atom-water radial distribution functions confirmed these shell structures around polar or charged atoms and apolar ones.

Powerful new software for the simulation of WAXS and SAXS diagrams

The study of poorly crystallized compounds by X-ray powder diffraction is sometimes very difficult because structural information is not easily obtained. One way to determine these structural data is to test more or less complex crystallographic models and compare simulated wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) spectra with experimental ones. This paper presents new software for the simulation of X-ray patterns. Three modules are available; PRECRAY, SIMVAX and SIMVAX1. PRECRAY allows the definition of the structural model for the solid and different conditions for the calculation of the X-ray diagram, such as the type of anode and the angular domain. SIMVAX and SIMVAX1 calculate all the interatomic distances according to the Debye formula; the contributions of the different kinds of atoms may be displayed. Applications to different sorts of material are presented; the influences of several kinds of crystallographic defects on the X-ray diagrams have been studied. The relevance of this simulation approach to the interpretation of small-angle X-ray scattering patterns, by the generation of complex models of particles and calculation of the scattered intensity, is also presented.

Monte Carlo simulation of epitaxial growth in MBE and ALE mode

A Monte Carlo based method for simulation of epitaxial crystal growth is reported. The improvement of our concept in comparison to the simulations based on the solid on solid model is the possibility to simulate crystal growth without the assumption of a discrete crystal lattice structure. Our model requires only an assessment for the Hamiltonian. The method is used for studying growth in a two-dimensional cross section through the substrate and the epilayer with a width of up to 80 atoms. Two different potentials can be chosen for the simulations: (a) Lennard-Jones and (b) directional Lennard-Jones to describe covalent systems. In addition to the selection of the potential, we can set the following parameters for different kinds of atoms: radii of the potential minimum, binding energies between the atoms for all possible pairings, growth temperature and substrate structure. The epitaxial growth of strained layers was simulated to study the incorporation of dislocations at the interface. Island and layer growth in the nucleation stage were investigated by selecting different binding energies of the epilayer atoms to the substrate atoms. As further extension of our MC calculations we allowed the reevaporation of particles which is essential for the simulation of atomic-layer epitaxy.

Probability wave theory of the atomic configuration for multicomponent crystal structures and its application to the ordered structure of complex perovskite materials

The probability wave of the atomic configuration is used to study the occupation probabilities of different kinds of atoms at lattice sites in multicomponent solid solutions and predict the crystal structures of multicomponent solid solutions. The theory of the probability wave of atomic configuration is applied to study the atomic configuration of the most stable superlattices formed in the complex perovskite structure. Seven basic ordered structures are deduced theoretically; these have been compared with the experiment results.

PROBABILITY WAVE THEORY OF THE ATOMIC CONFIGURATION FOR MULTI-COMPONENT CRYSTAL STRUCTURES

It is suggested that there is a kind of probability wave of the atomic configuration p(R)in crystals that can be deduced from the eigenequation of the ionic interaction Hamiltonian,and it has definite physical meaning. By use of the probability wave of the atomic con-figuration, the occupation probabilities of different kinds of atoms at lattice sites in multi-component solutions can be studied, and the crystal structures of multi-component solutionscan be predicted.

Kinetics of associative desorption from the dissolved state: phenomenon of the acceleration of the desorption and effect of flux “breaking”

The theory of associative desorption of different kinds of atoms from a dissolved state into the vacuum has been developed. The desorption character has been shown to be determined unambiguously by the ratio of the number of dissolved monolayers Γ of each of the components to their critical number Γ ∗, and by the ratio of the diffusion coefficients: p=DADB. We construct diagrams of the regions in which the kinetics are controlled by diffusion of A atoms (1), B atoms (2) or by their recombination (3). A new type of effect — the phenomenon of acceleration of desorption — has been predicted. The effect is found to be as follows: in the range Δ=(ΓA−ΓB)Γ∗ above some critical value Δc(p)(p<1) the rate of variation of the desorption flux grows drastically in the process of the 1 → 2 + 3 transition. In the range Δ > δd > Δc the acceleration is of stepwise character (diffusion → diffusion ty transition) and at p ⪡ 1 it is accompanied by an “instantaneous” vanishing of the flux from the sample (flux “breaking”). This effect is related to the nontrivial behaviour of the surface concentration of the slowly-diffusing component.

Contribution of Atomic-Level TEM to Resolution of Structure

Atomic Resolution Electron Microscopes are now producing useful results in many fields of science and technology. This success was obtained not only by the improvement of resolution of TEM but also through the developments of theories and experiments of diffraction crystallography, image formation and recording technique over the past 44 years. Boersch (1946-47) discussed the image of atoms and crystals by the phase object approximation; the image contrast is due to the phase shift of electron waves passing through them. Scherzer (1949) discussed the effect of the phase shift by the electron lens and proposed the phase contrast transfer function. He pointed out that the key to observe images of single atoms is by contrast enhancement, which might be possible by dark field images if resolution is improved. This proposal was attempted by improving the resolution using tilted-beam dark fields (Th atoms, Hg &amp; Pt atoms 1971). Crystal lattice fringes were observed by Menter in 1956 who discussed the contrast by kinematical theory. The interpretation of lattice images by dynamical theories were carried out by Hashimoto et al. (Bethe theory) and Cowley (Multi-slice theory) in 1958-60 and noted the correct position of atoms is in neither black fringe regions nor white ones when the images are taken in the Bragg reflecting position. However Miyake et al in 1964, showed that black or white contrast peaks appear at atom positions if the crystal is in symmetry position. Two dimensional lattice structure images were first photographed in 1970-71, which stimulated strongly the application of electron microscopy to materials science, crystallography and engineering at many laboratories such as the National Center at Berkeley. For the identification of correct position of atoms, two types of image contrast calculation have been proposed (e.g. multi-slice 1972, Bethe method 1975). The partially coherent theory originally developed in optics was introduced into the contrast calculation in 1979-80. Around this time, many observations of defect structures have been carried out, some of which are shown in Table 1. In situ observation of moving atoms and atom columns in molecules and crystals have been carried out in 1978 by using a TV system, which enable us to see the transition phenomena with a speed of 1/30 or 1/60 seconds. More recently atomic imaging at 1.8 A-1.6 A played an important role in the structure research of new materials and phenomena such as superconducting materials (metallic A15, 1989, ceramic high Tc, 1988), metal ceramic interface (Nb/Al2O3 1984), superstructure (GaAs/AlAs/GaAs 1985) quasi crystals (1987) etc. Many subsequent observations are presented in this congress. In spite of these developments, there are still some problems to be solved, e.g. imaging of atoms in the correct positions and the identification of different kinds of atoms in the materials especially with unknown crystal structure.

Pseudo-weak-phase-object approximation in high-resolution electron microscopy. II. Feasibility of directly observing Li+

Abstract Images of Li2Ti307 were calculated with the multislice method and observed with a JEM-200CX electron microscope. Both calculated and observed images show that Li ÷ ions can be seen intuitively under an optimum imaging condition as is predicted by the theory of the pseudo-weak-phase-object approxi- mation. 1. Introduction In the earlier papers in this study (Li & Tang, 1984, 1985) the dependence of image contrast on crystal thickness was discussed. When the crystal is too thick to allow the weak-phase-object approximation (WPOA) to hold, it was shown that with increasing crystal thickness the image contrast of light atoms increases more rapidly than that of heavy atoms. Hence, crystals too thick for weak-phase-object approximation to hold but thinner than the critical thickness have been treated as pseudo-weak phase objects. Under the optimum imaging condition the image intensity can be expressed as I(r)= 1- 2o'~o'(r). (1) Here ~o'(r) is the modified projected potential distri- bution function (PPDF) which can be treated as the PPDF of an imaginary isomorph of the examined crystal where real atoms are replaced by imaginary ones. The constituent heavy and light atoms of the imaginary isomorph are respectively lighter and heavier than those of the real crystal. This indicates the possibility of preferentially revealing different kinds of atoms by choosing observation regions of different thicknesses in the crystal examined. It was found by the HREM (high-resolution elec- tron microscopy) observation on the mineral cebaite BasCe2(CO3)sFz that within a certain range of crystal thicknesses the image contrast is sensitive to the posi- tions of light atoms (C, O, F) and hence the positions of these light atoms were determined by HREM (Li * Projects supported by the Science Fund of the Chinese Academy of Sciences. t Present address: Beijing Electron Microscope Laboratory, PO Box 2724, Beijing, China.

Electron correlations in molecules. II. General trends derived for isoelectronic series

We investigate general features of electronic correlations in four series of small isoelectronic molecules (partially model systems) containing hydrogen and first-row atoms. The calculations are done by combining a semi-empirical INDO scheme for the SCF part with the local ansatz for the calculation of interatomic correlations and an atoms-in-molecule approach for the determination of intraatomic correlation energies. The latter ones turn out to depend in the simple way on the mean number of electrons and the hybridization of an atom, but only weakly on the nuclear charge. We discuss how the individual (intra- and inter-bond) contributions to the interatomic correlation energy depend on the bond polarity, type of bonding and different kinds of atoms involved. Furthermore we characterize the suppression of charge fluctuations by interatomic correlations by means of a correlation strength parameter.

Calculating coordination numbers from radial-distribution peak areas in x-ray diffraction from a system containing atoms of different kinds

Calculating coordination numbers from radial-distribution peak areas in x-ray diffraction from a system containing atoms of different kinds

Shape of the peaks in radial distribution functions obtained from x-ray diffraction experiments in systems containing atoms of different kinds

Shape of the peaks in radial distribution functions obtained from x-ray diffraction experiments in systems containing atoms of different kinds

On a model structure for amorphous solids: A working approach to the structure of a perfect amorphous solid

According to the arguments by Sadoc a model structure for amorphous solids is defined, on the basis of the dense random packing (DRP) model, as a tetrahedral packing of hard spheres in a curved non-Euclidean space followed by a mapping onto the linear Euclidean space. The mapping introduces an equivocalness into the topology of the DRP, of which the nature is the central problem for the description of the model structure. The energetic implication of the DRP is given by a nil excess entropy. This definition of the DRP as a model structure for amorphous solids can be verified through the observation of densities, density fluctuations, mean coordination numbers, excess entropies, etc. Structural and thermodynamical changes of metallic glasses due to particle bombardments and cold works are compiled, and are reasonably interpreted with a reference to the DRP as defined above, suggesting the adoption of the DRP as a model structure for amorphous solids with single kinds of atoms. The model is extended to systems of different kinds of atoms with chemical orders. The extension is feasible by defining the structure units, which materialize the chemical order, and which are arranged as topologically equivalent to the hard spheres in the DRP. The arguments are verified by the effects of neutron irradiation on silica glass. The model structure here defined proves to work as a reference when an amorphous material is to be characterized, just as does the model of a perfect crystal for the characterization of crystalline materials.

On order and complexity. II. Application to chemical and biochemical structures

The organized complexity of a variety of chemical and biochemical compounds is estimated by the method outlined in the first paper of this series. The structural formulas of these compounds are treated as labelled graphs, the labels of the vertices being the different kinds of atoms, and those of the edges being the different kinds of bonds. Such a graph is coded for by a linear sequence of symbols which in turn is described as short as possible by means of additional symbols. The length of the shortest possible description of a structural formula obtained in that way is taken as a measure for the organized complexity of the underlying compound. Upper limits for the organized complexities of protein and RNA molecules are also estimated.

Adsorption ensembles and catalysis: a mathematical method

Adsorption and subsequent catalytic reactions of large molecules (e.g. higher hydrocarbons) usually require an ensemble of adsorption sites, i.e. a number of surface atoms being of the active species. The probabilities of certain ensembles (bridge sites and triangles) on a plane triangular lattice are calculated for fixed surface concentration of one atomic species depending on the interactions between different kinds of atoms. In a similar way the surface coverage of interacting adsorbed atoms on a perfect triangular lattice of adsorption sites is investigated with respect to its dependence on the partial pressure and the interaction energies. In both cases ordered surface structures can be found; the conditions of their formation and their consequences are pointed out.

Thermodynamic investigations on the formation and decomposition of metallic glasses

Metallic glasses usually can easily be formed in systems which are characterized by a strong interaction between atoms of different species, this fact leading to a more or less ordered distribution of the different kinds of atoms in the melt. Taking the gold-germanium system as an example, the nature of this short-range order, its concentration and temperature dependences and its influence on the formation of metallic glasses are discussed on the basis of mixing enthalpies. The relationship between the interatomic and the glass-forming ability has been used to discover a series of new metallic glasses, the main component of which is an earth alkaline metal. In order to produce these glasses a new method of splat cooling was developed. Furthermore, the energetics and kinetics of the crystallization of metallic glasses are discussed. As an example, the crystallization of a MgGa glass into a metastable intermetallic compound is considered.

Determination of Localized Magnetic Moments in FeCrAl Alloys and the Electron Structure

AbstractThe localized magnetic moments of Fe and Cr are determined by combination of saturation magnetization measurements and magnetic diffuse scattering. Power series characterizing the interactions between the different kinds of atoms in the alloys are chosen to describe the concentration dependence of the magnetic moments. The different terms are discussed on the basis of band structure models valid for dilute alloys taking into account their modification by impurity interactions.

Effect of short-range order on thermo-emf of binary alloys

The thermo-emf of binary disordered alloys is calculated, taking into account correlations in the distribution of atoms of different kinds over the lattice. It is shown that the presence of short-range order in the alloys may lead to a nonlinear temperature dependence of the thermo-emf and cause a change in its sign.

The simulation of backscattering, part I: Description of the simulation model and program

A Monte Carlo simulation program (BACKS) has been written in such a way as to be efficient for the generation of backscattering data for particle energies of approximately 1 keV to 2 MeV. The program is several thousand times as efficient as conventional programs in generating backscattering data at higher energies due to an information return analysis routine involving particle generation over selected sub-areas of the crystal face. The program takes into account energy losses and thermal motion. It allows for a nearly complete range of beam-to-crystal-face angles. Most crystal forms can be used as well as crystals containing several different kinds of atoms. The data output includes a depth of information distribution, energy loss spectrum, a number of collisions, distribution, and a backscattering pattern. Sample results of α-particles in a gold monocrystal are given.