Relative Positions Of Atoms Research Articles

Radial distribution of carbon atoms in crystalline and molten polyethylene from x-ray diffraction

X-ray diffraction data for crystalline and molten polyethylene are analyzed to yield radial distribution functions. Use of scattering factors for CH2 groups in the data reduction yields unambiguous, model-independent information on the relative positions of carbon atoms. Direct comparison of the atom pair distribution functions rules out extended chain structures in the melt.

EXAFS Studies of Semiconductor Microstructure

Extended X-Ray Absorption Fine Structure (EXAFS) is a technique capable of probing local structure in condensed- matter systems, determining distances, coordination numbers, and species of atoms near an x-ray excited atom. Further, this information is obtained separately for each constituent atomic species. EXAFS can be applied equally well to crystals, alloys, or even systems having no long-range order such as liquids, gases, and amorphous materials.Here we review EXAFS spectroscopy and its use in studying semiconductors. First, we show EXAFS in the study of impurities to determine the local environment of Fe implanted in Si. Second, we show how EXAFS can be used to determine local structural distortion in diluted magnetic semiconductors. We next demonstrate that EXAFS can determine the nature of the phase transition in ferroelectric Pb1-x Gex Te, where the transition is revealed to have an order-disorder character in contrast to GeTe where the transition is purely displacive. The theme of these examples is that EXAFS can determine the relative positions of atoms in condensed matter and that the short-range order as determinedn by EXAFS often differs from the long-range order or theoretical expectations.In the EXAFS technique, one measures the x-ray absorption coefficient as a function of x-ray energy. The absorption spectrum shows a number of x-ray edges corresponding to the threshold of excitation of core electrons from the sample's various atomic species. At energies above each of these edges, oscillations in the absorption coefficient are due to backscattering of the photoelectron from the neighboring atoms.

Estimate of repulsive interatomic pair potentials by low-energy alkali-metal-ion scattering and computer simulation.

Low-energy ion scattering is used in combination with computer simulation to study the interaction potential between 600-eV potassium ions and atoms in metallic surfaces. A special algorithm is described which is used with the computer simulation code marlowe. This algorithm builds up impact areas on the simulated solid surface from which scattering cross sections can be estimated with an accuracy better than 1%. This can be done by calculating no more than a couple of thousand trajectories. The screening length in the Moli\`ere approximation to the Thomas-Fermi potential is fitted in such a way that the ratio between the calculated cross sections for double and single scattering matches the scattering intensity ratio measured experimentally and associated with the same mechanisms. The consistency of the method is checked by repeating the procedure for different incidence conditions and also by predicting the intensities associated with other surface scattering mechanisms. The screening length estimates are found to be insensitive to thermal vibrations. The calculated ratios between scattering cross sections by different processes are suggested to be sensitive enough to the relative atomic positions in order to be useful in surface-structure characterization.

Structural aspects of bonding in silicon carbide polytypes: ab initio MO calculations

The series of polytype structures of silicon carbide was modelled using Si(CH 3) 4(SiH 3) 4 molecules with C 3 v symmetry. The Si-C bond lengths and angles were calculated using ab initio molecular orbital methods where the external dimensions of the molecules were constrained to values equivalent to the lattice parameters of the polytypes. Second order coordinations of hexagonal and cubic stacking sequences in the crystal structures were simulated by the relative positions of hydrogen atoms. The 6–31G basis level bond lengths and angles are shown to conform to linear relationships with respect to the lattice parameters but differ between the two second order coordinations. Except for the perfect tetrahedral structure of 3C-SiC all vertical bonds in the [001] direction are longer than the lateral bonds inclined to the basal plane. It is shown that the charge on the central atom is independent of the c a ratio of lattice parameters but is slightly different between hexagonal and cubic configurations.

Angle Resolved Ion-Electron Spectroscopy (ARIES), a new technique to study surface structural features

Abstract Angle Resolved ion-Electron Spectroscopy (ARIES), combined with Low Energy Ion Scattering (LEIS), is presented as a new technique with a wide applicability to study solid surfaces. Energy and angular distribution of secondary electrons emitted due to the ionization of (ad)-sorbates on a Cu (001) surface upon bombardment with 10 keV He+ are examined. When sulphur atoms—diffused from the bulk to the surface upon a 600°C anneal—are involved in the emission process, a strongly anisotropical emission is found. The anisotropy is related only to the incoming beam direction and appears to be fully independent of the copper target orientation. Some points of importance for a theoretical description are discussed. The anisotropy effect can effectively be used to determine the relative position of contaminant and substrate atoms in the uppermost surface layers. Thus, it is found that (1) sulphur atoms diffused to positions just below the surface occupy substitutional sites and (2)in this situation surface r...

Analysis of Illicit Drug Exhibits by Hydrogen-1 Nuclear Magnetic Resonance Spectroscopy

Abstract Positive identification of most illicit laboratory exhibits routed to the Special Testing and Research Laboratory of the Drug Enforcement Administration (DEA) for the identification of unknown constituents has been rapidly achieved with the aid of a nuclear magnetic resonance (NMR) spectrometer. This technique often provides valuable quantitative data as well. Magnetic resonance spectroscopy of the hydrogen atom provides information on the chemical nature of a single hydrogen atom (or group of atoms) within a molecule, the number of hydrogen atoms in a particular group, and the number and relative position of hydrogen atoms in neighboring groups.

Application of NQR double-resonance technique to a relative assignment of next-nearest neighbors within complex ions. Aluminum and bromine in KAl 2Br 7

The 1 r 6 dependence of the dipolar coupling can be used in the nuclear quadrupole double-resonance technique to gain information on relative positions of atoms within complex ions and molecules. With 27Al &.zharr; 81Br NQDR the terminal and bridging bromine atoms could be located relative to the two Al atoms for the anion [AI 2Br 7] ⊖ in KAl 1Br 7. Application of the Townes-Dailey theory leads to a reasonable qualitative bond model for the ion IAl 2Br 7] ⊖.

The Use of Diffraction Pattern Information In Stem

For each position of the incident beam in a STEM instrument, a convergentbeam diffraction pattern is produced in the detector plane. Except in the idealized, impracticable case of single isolated atom specimens, the intensity distribution in this diffraction pattern contains information regarding the relative positions of atoms within the volume element of the specimen illuminated by the beam. If this intensity distribution is recorded, for example by use of a two-dimensional detector array (Fig. 1) rather than a single detector which integrates the intensity, the additional data available should allow additional information to be derived concerning the specimen. When radiation damage is not a problem, this should allow an improvement in resolution. For radiation sensitive specimens, this should allow the incident beam intensity to be spread over a larger area so that radiation damage is reduced.

Electron-diffraction and electron-microscopy methods for the study of ordering in crystals

The diffraction of electrons is, in principle, a very sensitive probe of the relative atomic positions in ordered or disordered crystals. The full potential of electron diffraction and electron microscopy methods for the study of local atomic arrangements has not been realised because of the difficulties of conducting experiments under sufficiently well defined conditions and because of the complications introduced by the dynamical diffraction processes involved.

Determination of the electronic properties of carboranes, carborane anions, and metallocarboranes from fluorine-19 nuclear magnetic resonance studies

Abstract : The preparation of the carbon substituted m- and p- fluorophenyl derivatives of a series of carboranes, carborane anions and metallocarboranes allowed the electronic properties of these species to be determined from 19F nmr chemical shifts and the method of Taft. Species examined in this manner include the 1,2- and 1,7- isomers of the B10C2H12 icosahedral carboranes and the corresponding B9C2H12(-) and B9C2H11(-2) ions as well as the 1,8-B9C2H11 carborane and selected transition metal derivatives of the (3)-1,2- and (3)-1,7-B9C2H11(-2) ions. In several instances large and negative Sigma(R) values were observed which illustrates ground state electron donation from the carborane moiety to the substituted phenyl ring by a pi-mechanism. The Sigma(1) values were affected by both the relative carbon atom positions in the carborane fragments and the presence of a negative charge delocalized in those fragments. (Author)