Structure analysis of the KBr(100) surface An investigation with a new method for surface analysis of insulators

The surface structure of highly insulating KBr(100) was investigated by a new technique, impact collision atom scattering spectroscopy with detection of neutrals (NICASS). By using neutral atoms in the scattering experiment as projectiles [different from conventional ion scattering (ISS)], uncertainties in the experimental data due to surface charging effects are avoided. As a result, a surface structure model for KBr(100) is presented. By comparing the experimental scattering information with computer simulated data, an inward relaxation in the first layer has been deduced. The relaxation is different for the two components K and Br: K is shifted by 0.5 Å and Br by 0.3 Å, resulting in a surface rumpling of 0.2 Å.     

Zeeman scanning of alkaline-earth absorption line profiles in flames at atmospheric pressure

Absorption line profiles of the resonance lines (¹P₁-¹S₀) of calcium, strontium and barium were measured in laminar, premixed shielded acetylene-air flames at atmospheric pressure by making use of a Zeeman scanning technique. The central wavelenght of the narrow atomic emission line in a low-pressure discharge containing Ca, Sr and Ba, respectively, was changed by less than ±0.2 Å with a variable magnetic field.

Values for the line shift, the asymmetry ratio, collisional half-width and damping parameter “a” were derived from these profiles at a flame temperature of 2275 ±10°K. Effective values for collisional cross sections were calculated.

Values for the line-damping parameter (and effective collisional cross sections) were determined under the same experimental conditions by using a variant of the “curve-of-growth” method. The results derived from the two methods were found to be consistent.

The new experimental results were compared with recent literature data. An attempt was also made to fit our experimental data to the Lindholm-Foley line-broadening theory for a Lennard-Jones type interaction; we have derived values for the constants C₆ and C₁₂ occuring in this potential.     

On the “quantum rainbow” in surface scattering

In this letter we propose a new method to compute the scattering amplitudes of atoms from a corrugated wall model of a crystal surface. The method method requires reasonable computing time and converges rapidly to the solution of the scattering amplitudes (within 0.01% in each beam and satisfying unitarity) for any corrugation such that kζ₀ < 4.5 and ζ₀ < 0.35 Å, where k is the magnitude of the incident wave vector and ζ₀ the amplitude of the corrugation. These conditions for kζ₀ and ζ₀ are satisfied for any crystal surface and light scattering atoms (H and He) at thermal energies which makes the method appropriate for the system He, LiF for which results are presented comparing them with recent experimental data.     

Leed theory for incommensurate overlayers: Application to graphite on Pt(111)

A dynamical LEED analysis is applied to study the structure of incommensurate overlayers. The previously described BSN method is extended to the incommensurate case, providing an efficient theoretical scheme for calculating the required LEED beam intensities. This method is used to investigate the structure of an incommensurate graphite overlayer on the Pt(111) surface by analyzing the specular diffraction beam intensities at various angles. The measured LEED intensities are well represented by a surface model consisting of a graphite layer 3.70 ± 0.05 Å above the Pt(111) surface, supported by at least a partial layer of “intercalated” carbidic carbon atoms chemisorbed in three-fold hollow sites 1.25 ± 0.10 Å above the Pt(111) surface. A “data subdivision method” is applied in the R-factor analysis to distinguish between the different R-factor minima.     

Allowance for the Coulomb interaction in the framework of an algebraic version of the resonating group method

The Coulomb proton interaction is taken into account in the problem of light atomic nucleus scattering using an algebraic version of the resonating group method. Specific formulas are obtained for calculating the elastic scattering phases. The elastic pα scattering phases, the positions of resonances and their widths are calculated for some NN potentials. Theoretical values of the quantities under consideration adequately describe experimental data.     

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.     

Hot electrons in silicon dioxide: Ballistic to steady-state transport

Hot electron transport in silicon dioxide is examined with emphasis on current experimental and theoretical results. For oxide layers thicker than 100 Å, steady-state transport has been shown to control the carrier flow at all fields studied. The transition from a nearly thermal electron distribution at electric fields less than approximately 1.5 MV/cm to significantly hot distributions with average energies between 2 and 6 eV at higher fields of up to 16 MV/cm is discussed. The significance of nonpolar phonon scattering in controlling the dispersive transport at higher electric fields, thereby preventing runaway and avalanche breakdown, is reviewed. The transition from ballistic to steady-state transport on very thin oxides layers of less than 100 Å in thickness and the observation of single phonon scattering events are also discussed.     

An application of the multichannel cluster model theory to the elastic proton-α scattering

In the framework of the cluster model reaction theory a calculation for the elastic α-p scattering below 30 MeV was performed. The nucleon-nucleon potential, which contains spin orbit and tensor forces, has been succesfully used in reaction calculations for other light nuclei. The calculated α-p cross section and polarization values are in quantitative agreement with experimental data.     

Surface deposition and coagulation efficiency of combustion generated nanoparticles in the size range from 1 to 10 nm

The size distribution of the nanoparticles formed in premixed ethylene–air flames and collected thermophoretically on mica cleaved substrates is obtained by atomic force microscopy (AFM). The distribution function extends from 1 to about 5 nm in non-sooting flames and in the soot pre-inception region of the richer flames, while it becomes bimodal and larger particles are formed in the soot inception region of the slightly sooting flames. The distribution is compared with the size distribution of nano-sized organic carbon (NOC) and soot particles, obtained by “in situ” multi-wavelength extinction and light scattering methods. The deposition efficiency is estimated from the differences between these two size distribution functions as a function of the equivalent diameter of the nanoparticles. Furthermore, the coagulation coefficient of particles in flame is obtained from the temporal evolution of the number concentration of the nanoparticles inside the flames. NOC particles, which are rapidly produced in locally rich combustion regions, have peculiar properties since their sticking coefficient both for coagulation and adhesion result to be orders of magnitudes lower than that expected by larger aerosols, like soot particles. The experimental results are interpreted by modelling the van der Waals interactions of the nanoparticles in terms of Lennard-Jones potentials and in the framework of the gas kinetic theory. The estimated adhesion and coagulation efficiencies are in good agreement with those calculated from AFM and optical data. The very low efficiency values observed for the smaller particles could be ascribed to the high energy of these particles due to their Brownian motion, which causes thermal rebound effects prevailing over adhesion mechanisms due to van der Waals forces.     

Reorganization of the cluster state in a C60/N-Methylpyrrolidone/water solution: Comparative characteristics of dynamic light scattering and small-angle neutron scattering data

Data on dynamic light scattering from cluster solutions of C₆₀ fullerenes in N-methylpyrrolidone (NMP) and its mixture with water are analyzed. Initial C₆₀/NMP solutions kept for two weeks (i.e., fresher than those analyzed previously) are considered, where the effect of cluster-state reorganization after adding water is recorded more reliably. Based on the size-distribution functions of the clusters, obtained from dynamic light scattering data, model curves of small-angle neutron scattering are calculated and compared with the experimental data.     

Determination of the Disperse Composition of a PbO Suspension Containing Aggregates of Particles of Lamellar Shape by the Laser-Polarimetry Method

The results of measurements of the scattering matrix at a wavelength of 0.63 μm in the range of scattering angles of 10°–155° are presented for an aqueous suspension of lead oxide containing particles of plate form and their aggregates of monomers with dimensions of ~5 nm. The results of the measurements are compared with the results of calculations for axially symmetric scatterers (ellipsoids of rotation, cylinders). It is shown that the presence of aggregates affects the scattering properties of such a medium. The results of reconstructing the distribution of particles of a disperse medium in sizes from the measurements data of the scattering matrix are presented. The reconstruction of the distributions was carried out by solving the problem of optimizing the sum of the squared deviations of the experimental and calculated values of matrix elements in the framework of the model of axially symmetric scatterers. It is shown that the distribution of particles by sizes is more accurately reconstructed by minimizing the sum of the squares of the deviations for the sum of the diagonal elements. The obtained distribution is compared with the distribution measured by the method of dynamic light scattering.     

Determination of zone-boundary magnon energy and damping in RbMnF3 by means of light scattering experiments

New experimental data of two-magnon Raman scattering on the cubic antiferromagnet RbMnF₃ are used to obtain information on the single spin dynamics in all the ordered phase. The comparisons with the existing second order theory and with the neutron scattering data shows that the light scattering gives correct values for energy and lifetime of the zone boundary magnetic excitations.     

Structural properties of diamond fine particles and clusters prepared by detonation and decomposition of TNT

The thermal parameter B for three different particle sizes of diamond samples (bulk powder 1–4 μm, fine particle 144–195 Å and cluster 55–61 Å) was determined by the grazing incidence X-ray diffraction method. The values of B were found to be in the range 0.50–0.70 Å² for particles in the size range 195–55 Å and 0.27 Å² for 1–4 μm. All of them are larger than that of diamond bulk. A clear size dependence of B, increasing with decreasing particle size, was found. By analysing X-ray diffraction data at several temperatures the magnitude of B was found to be due to B_S (static part) instead of B_T (dynamic part). The average B_S values obtained were 0.04 Å², 0.19 Å² and 0.27 Å² for bulk powder, fine particle and cluster samples respectively. Ultrahigh resolution transmission electron microscope (TEM) observation confirmed the presence of strain, distortion, roughness and dislocation lines in many particles. TEM images of particles indicate that the clusters were not spherical in shape; they were mostly cubiform and some were truncated prism-like polyhedral. The present study reveals that the B_S component is responsible for the large B value in diamond fine particles and clusters. No clear surface local atomic distortion was found in the particles.     

An analysis of pion-nucleus scattering lengths within an α-cluster model

Pion-nucleus s-wave scattering lengths are calculated from a multiple scattering series in terms of the π-α scattering length. Agreement on the level of 10–20% with the values obtained from mesic-atom data is achieved for all nuclei with A ≦ 20. This lends weight to the speculation that pion absorption on light nuclei takes place primarily on α-particle substructures.     

Crystal lattice dynamics of ZnGeP2 and AgGaS2 in hard ion model

A calculation is made of the long-wavelength phonon spectrum of ZnGeP₂ and AgGaS₂ in the hard ion model. The parameters of short-range forces and dynamic charges are found by comparing the theory with the experimental data on infrared absorption and Raman scattering of light. The elastic and piezoelectric constants are calculated. The available experimental data are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–10, May, 1980.     

Optical properties of grooved silicon microstructures: Theory and experiment

The reflection spectra of grooved silicon structures consisting of alternating silicon walls and grooves (air channels) with a period of a = 4–6 μm are studied experimentally and theoretically in the mid-IR spectral range (2–25 μm) upon irradiation of samples by normally incident light polarized along and perpendicular to silicon layers. The calculation is performed by the scattering matrix method taking into account Rayleigh scattering losses in a grooved layer by adding imaginary parts to the refractive indices of silicon and air in grooved regions. The experimental and calculated reflection spectra are in good agreement in the entire spectral range studied. The analysis of experimental and calculated spectra gave close values of the effective refractive indices and birefringence of the studied structures in the long-wavelength spectral region. The values calculated in the effective medium model in the long-wavelength approximation (λ ≫ a) gave considerably understated values. The obtained results confirm the efficiency of the scattering matrix method for describing the optical properties of silicon microstructures.     

Determination of the K adsorption site on Fe(1 1 0) with XSW

We have used X-ray standing waves (XSW) in near normal incidence to determine the K–Fe bond length and the adsorption site of K at the saturation coverage at room temperature on the Fe(1 1 0) surface. Three different scattering geometries were used to enable the determination of the adsorption site by triangulation. From the results we conclude that the potassium atoms adsorb in a distorted hexagonal overlayer. The Fe–K distance, as determined from the measurements in the (2 2 0) Bragg reflection, is 3.4±0.2 Å. The long bridge site seems to be the preferred adsorption site for the potassium atoms in the distorted hexagonal overlayer. This geometry not only fits all the XSW data, but also explains all spots in the LEED pattern without the need to introduce multiple scattering. Comparison of the measured and simulated XSW data, based on the distorted hexagonal overlayer, enables a more accurate determination of the Fe–K bond length to 3.36±0.14 Å. This corresponds to a potassium hard sphere radius of r_K=2.12±0.14 Å. This radius is among the largest reported for potassium on a metal, which is attributed to the high coverage and coordination of the K atoms in this overlayer arrangement.     

Interferenzen bei atomaren Stoßprozessen und ihre Interpretation durch ein modifiziertes Lennard-Jones-Potential

The influence of the shape of the interatomic potential on the total and differential scattering cross section, with special consideration given to interference effects, was numerically studied with the help of a 5 parameter modified Lennard-Jones potential. It is shown that the experimental data for the systems Na-Kr and Na-Xe, for which precise measurements are available, can be well reproduced with the use of such a potential. The absolute value and velocity dependence of the total scattering cross section as well as rainbow scattering and rapid oscillations in the differential scattering cross section were all used in the comparison of the calculated with the experimental values. In addition, the modified potential has been used in a re-evaluation of recent experimental results for alkali metal —inert gas scattering.     

Non-nuclear maxima (NNM), symmetric and asymmetric charge distribution in solar grade Si and n-GaAs, using X-ray powder data

The symmetric and asymmetric charge distribution of bonding electrons in silicon and gallium arsenide has been studied using experimental powder X-ray data sets. The mid-bond electron densities from the origin are found to be 0.787 and 1.725 e/ų at 1.175 and 1.222 Å, respectively, for Si and n-GaAs. The effectiveness of powder data sets in precise bonding studies has been verified. Multipole analysis of the valence charge distribution has also been done. The thermal vibration parameters have been studied and presented for both these systems. The magnitudes of the non-nuclear maxima of both these systems are consistent with those obtained using single-crystal studies.