Spread Of Angles Research Articles

Dislocation boundaries—the distribution function of disorientation angles

In dislocation structures orientation differences arise across dislocation boundaries during plastic deformation. The distribution function of the disorientation angles related to dislocation boundaries has been determined empirically and is almost independent of experimental parameters such as material type, plastic strain, temperature and deformation conditions (Hughes et al., Phys. Rev. 81 (1998) 4664). In the present paper distribution functions are derived from geometrical considerations for quite general assumptions on the number of sets of parallel dislocations and their arrangement. The relation between the obtained distribution functions for the disorientation angles and general orientation distributions is elucidated. A comparison with the experimental results shows that the Rayleigh distribution obtained for an equivalent contribution from two dislocation sets with perpendicular rotation axes is most appropriate for describing the experimental data. For experimental distributions with a larger spread in the disorientation angles and relatively large deviations from a Rayleigh distribution a superposition of two Rayleigh distributions is suggested implying the presence of two different types of boundaries in a given structure. Finally, it is shown that an analysis of distribution functions of disorientation angles can further the understanding of deformation induced structural changes.

Raman spectra and structure of amorphous Si

In 1985, Beeman, Tsu, and Thorpe established an almost linear relation between the Raman transverse-optic (TO) peak width \ensuremath{\Gamma} and the spread in mean bond angle \ensuremath{\Delta}\ensuremath{\theta} in $a\ensuremath{-}\mathrm{Si}.$ This relation is often used to estimate the latter quantity in experiments. In the last decade, there has been significant progress in the computer generation of sample networks of amorphous silicon. Exploiting this progress, this paper presents a more accurate determination of the relation between \ensuremath{\Gamma} and \ensuremath{\Delta}\ensuremath{\theta} using 1000-atom configurations. Also investigated and quantified are the relations between the TO peak frequency and the ratio of the intensities of the transverse-acoustic (TA) and TO peaks, both as functions of \ensuremath{\Delta}\ensuremath{\theta}. As \ensuremath{\Delta}\ensuremath{\theta} decreases, the TA/TO intensity ratio decreases and the TO peak frequency increases. These relations offer additional ways to obtain structural information on $a\ensuremath{-}\mathrm{Si}$ from Raman measurements.

Improved modeling of system response in list mode EM reconstruction of Compton scatter camera images

An improved List Mode EM method for reconstructing Compton scattering camera images has been developed. First, an approximate method for computation of the spatial variation in the detector sensitivity has been derived and validated by Monte Carlo computation. A technique for estimating the relative weight of system matrix coefficients for each gamma in the list has also been employed, as has a method for determining the relative probabilities of emission having some from pixels tallied in each list-mode back-projection. Finally, a technique has been developed for modeling the effects of Doppler broadening and finite detector energy resolution on the relative weights for pixels neighbor to those intersected by the back-projection, based on values for the FWHM of the spread in the cone angle computed by Monte Carlo. Memory issues typically associated with list mode reconstruction are circumvented by storing only a list of the pixels intersected by the back-projections, and computing the weights of the neighboring pixels at each iteration step. Simulated projection data has been generated for a representative Compton camera system (CSPRINT) for several source distributions and reconstructions performed. Reconstructions have also been performed for experimental data for distributed sources.

Magnetic field draping around Mars: Mars Global Surveyor results

We investigate the contribution of the radial component of the magnetic field in the solar wind interaction with Mars. We use Mars Global Surveyor Magnetometer and Electron Reflectometer data during the Science Phasing Orbits to gain a qualitative understanding of the magnitude of the radial magnetic field from the solar wind draping around the dayside of Mars. Using only data far away from the strong crustal magnetic sources, we find that the angle the magnetic field makes to the horizontal within the martian ionosphere has an average closer to zero than that outside the ionosphere. However, we find a much larger spread in angles inside the ionosphere. In addition, we find that the angle of inclination increases with increasing altitude and local time, but more spatial coverage is needed to quantify these relationships.

Assessment of zinc finger orientations by residual dipolar coupling constants.

Residual dipolar coupling constants measured in anisotropic solution contain information on orientations between internuclear vectors and the magnetic field, providing long-range information that may help determine the relative orientations of distinct domains in biomolecules. Here we describe the measurement and use of residual dipolar coupling restraints in the refinement of the structure of the complex of DNA with three zinc fingers of transcription factor IIIA (TFIIIA), measured in a DMPC/DHPC bicelle solution. These dipolar restraints were applied on a variety of orientations of the zinc finger domains (derived from crystallography, previous NMR studies, and systematic modeling) in order to examine the validity and sensitivity of using residual dipolar splittings to study interdomain orientations. The spread in interdomain angles between zinc fingers is reduced from 24 degrees to 9 degrees upon incorporation of dipolar restraints. However, the results also show that the ability to determine relative orientations is strongly dependent on the structural accuracy of the local domain structures.

Observation of chloride-ion hydration in high-temperature liquid and supercritical water by spherical harmonic expansion analysis

The structure of the Cl− hydration shell in high-temperature (77 and 197 °C) and supercritical (375 °C) water at moderate densities (roughly twice the critical density of 0.356 g/cm3) as well as in water at an ambient temperature, is investigated by means of neutron diffraction with isotope substitution on Cl− in aqueous 3 and 9 m solutions of LiCl in D2O. These relatively high densities (compared to the critical density) are used here to maintain the salt solubility at a sufficiently high value that a measurable signal is obtained in the isotope difference. The data are subjected to an empirical potential structure refinement which enables individual site-site radial distribution functions to be estimated. The same structure refinement is also used to estimate the spherical harmonic coefficients of the Cl-water orientational pair correlation function, and these are used in turn to reconstruct the local environment around Cl− in solution. A marked disordering of the Cl− hydration shell is observed as the critical point is approached. The orientational correlation functions of the water-molecule dipole vectors around Cl− in ambient water show a strong peak at a distance of about 3.2 Å, and centered at an angle of about 126°±30°, with respect to the Cl-O(D2O) axis, confirming an almost linear Cl⋯D-O hydrogen bond but with a significant spread of orientations about this value. At the higher-temperatures the peak remains at about the same position, but becomes progressively weaker in amplitude and with a larger spread of angles, suggesting a gradual decrease in orientational correlation of the water molecules around Cl−. At the supercritical temperature, the peak is even weaker, and there is evidence for a broader hydration shell developing at a distance of r≈3.9 Å in which the water molecules are apparently randomly oriented: they apparently become more loosely packed than at lower temperatures, without being dispelled from the ion hydration shell.

Structural Refinement and Stability of Silicon XII

A new Si phase was obsd. using high-resoln. angle-dispersive x-ray powder diffractometry and the structure was solved and refined (no data). Si XII was reversible formed above 2 GPa via a 1st-order transition from Si III. It has a larger rhombohedral angle (110.07� at 8.2 GPa) than cubic Si III, which reduces the cell vol. by ?2% at the transition. The space group is R?3 with 8 atoms per cell. There is a wide spread of bond angles within the structure making Si XII the most complex, tetrahedrally-bonded cryst. phase of Si. Simulated transition mechanisms showed that the phase transition involves 1 bond breakage and 1 new bond formation per unit cell. [on SciFinder(R)]

Mode competition in fourth-harmonic magnicon amplifiers

In the magnicon RF amplifier, the drive and gain cavities are cylindrical deflection cavities which operate in a rotating TM/sub 110/-mode and spin up an electron beam to high transverse momentum. As a result of the rotating-mode interaction, the electron beam entry point into the output cavity rotates about the axis at the drive frequency. The gyrotron-like output cavity can be operated at m times the drive frequency by using a mode with an azimuthal index of m, as this mode rotates at m/sup -1/ times its RF frequency, thus maintaining synchronism with the electron beam. Previous frequency-multiplying magnicons have used m=2; in this paper it is shown that magnicons with m=4 may be practical, provided one also operates the output cavity at the m/2 harmonic of the cyclotron frequency. Operation at higher harmonics lowers the frequency of the deflection cavities allowing lower RF fields, reducing cavity breakdown problems; lower magnetic fields, reducing magnet cast and complexity; and a larger electron beam, relaxing beam quality constraints. On the other hand, higher order azimuthal-index magnicon modes interacting at higher order cyclotron interactions are subject to competition with nonsynchronous (gyrotron) modes and are more sensitive to electron beam scanning angle spread. A time-dependent multimode gyrotron code has been modified to examine competition in the output cavity between the phase-synchronous operating mode and other nonsynchronous modes which interact via the conventional gyrotron interaction. Calculations have been carried out for fourth-harmonic magnicons with TM/sub 410/ and TE/sub 411/ mode output cavities and include the effects of mode competition with nonsynchronous modes as well as a spread in entry-point angles of the scanning electron beam.

The solidification behaviour of Bi particles embedded in an Al matrix

This paper reports heterogeneous nucleation behaviour of embedded nano- and micron-scaled Bi particles in an aluminium matrix synthesized by rapid solidification and chill casting. It is shown that only a fraction of the nanoparticles that have a truncated octahedron shape show orientation relation with the matrix. The solidification of the embedded Bi melt was found to take place over a wide temperature range. The micron sized particles do not show any solidification peak while a broad shallow peak can be observed for nano-scaled particles. The observed broad differential scanning calorimetry exotherm has been deconvoluted into two peaks, one sharp and the other broad in shape. The exotherms are also simulated using classical theory of heterogeneous nucleation using a parametric approach and matched with the experimental one. It is shown that the broad peak can be best explained by a spread of contact angles during nucleation while the sharp peak can be fitted with a single contact angle. The latter is most likely associated with particles showing orientation relation and represents nucleation of Bi on the 111 plane of aluminium. Analysis of the present result emphasizes the important role of the local microscopic structures of the liquid-solid interface in promoting the catalytic nucleation efficiency of the Al matrix.

The crystal structure of [Cu(Ph 2bipy) 2][BF 4] · CH 2Cl 2 (Ph 2bipy = 6,6′-diphenyl-2,2′-bipyridine)

The crystal structure of the title compound has been determined. The normal four-coordinate geometry of [Cu(NN) 2] + cations (where NN denotes an α,α′-diimine chelate) is highly distorted, with a wide spread of bond lengths and angles, due to the presence of one strong intramolecular π-stacking interaction between a phenyl ring of one ligand and the coordinated bipyridyl fragment of the other.

The effect of varying substrate temperature on the structural and optical properties of sputtered GaAs films

Stoichiometric GaAs films have been prepared by RF sputtering over an appreciable range of substrate temperature Ts. The structural and optical properties of these films have been investigated by means of a variety of experimental techniques. Transmission electron microscope (TEM) experiments reveal that for Ts>or= approximately 70 degrees C the structure consists of nanocrystals embedded in an amorphous network. EXAFS, XPS and infrared experiments are consistent with each other, indicating that configurational disorder in the amorphous network of the GaAs films decreases as Ts is raised. Most of the disorder relates to bond-angle disorder; the spread in bond angle around Ga atoms is found to be less than that around As atoms for a given substrate temperature. The changes observed in the fundamental absorption edge with increasing Ts are discussed in relation to various possible defects in amorphous GaAs.

Passive gyrotron cavity loading and frequency shift

Abstract Measurements of change in Q and frequency shift are presented for the penultimate cavity in a 3-cavity gyroklystron under conditions where the cavity does not oscillate. This allowed detailed comparisons to be made with linearized theory, over a wide range of beam currents and DC magnetic fields. The theory was formulated to take into account the effects of cut-off beam tunnels on the effective length of the cavity, the geometrical beam filling factor, the effect of a spread in beam pitch angle, and the effect of the cavity RF magnetic fields. Best fits between the data and the theoretical curves for change in Q and frequency shift were obtained for beam values in good agreement with those predicted from a modified Herrmannsfeldt code. Calculations of oscillation starting currents for TE01, TE011, TE012, and TE rectangular013 cavities as a function of the cavity detuning parameter show that serious errors can occur if one neglects the cavity RF magnetic fields.

Structural investigation of silica gel films by infrared spectroscopy

Fourier transform infrared absorption spectroscopy has been utilized to characterize the structure of porous silica gel films, both deposited on c-Si substrates and free standing. The films were either dried at room temperature or subjected to partial densification at 400–450 °C. The spectra of the gel films are compared to those of thermal SiO2 grown on c-Si and to Kramers–Kronig analysis of the reflection spectra of bulk SiO2 gels and v-SiO2. The gel films show small frequency shifts compared to the latter spectra and they also exhibit new bands due to the presence of OH groups, although very little molecular water or residual organic species were found. The results are interpreted in terms of the gel structure. Compared to the thermal oxide, the sharp peak near 1070 cm−1 is narrower for the gels and the spread in intertetrahedral angles is estimated at 24° and 27° for room temperature dried and partially densified gels, respectively, compared to 33° for the thermal oxide. This is in agreement with a state of strain in the Si—O—Si bridges of the gels near the surface of the pores. The gel films have also a stronger shoulder near 1200 cm−1, whose intensity decreases with heat treatment of the gel. Its nature is related to the longitudinal optical (LO) component of the high-frequency vibration of SiO2 and it is suggested that the gel pores may account for the activation of the LO component.

Micro-Ellipsometric Measurements of Strained Layer Structures

ABSTRACTEllipsometric measurements are extremely sensitive (on the monolayer scale) to layer thicknesses and surface or interfacial roughness. However, lateral resolution is poor, because of the need for a highly collimated ( ≤0.05° ) and therefore large diameter (typically 1 mm) optical beam. Many materials are inhomogeneous on a smaller lateral scale, in which case conventional ellipsometry simply measures some average value. Focusing the beam leads, for an incoherent light source, to an uncollimated beam and a consequential spread in the angle of incidence, which in turn degrades the sensitivity. We describe here a technique for focusing a coherent, Gaussian laser beam to a minimum spot size of about 30μm × 130μm, while maintaining a negligible effective spread in angle of incidence. The technique was used to measure semi-insulating (S.I.) GaAs, laser annealed ion implanted GaAs, and InIGa1−xAs strained layer structures.

Radiation guiding and efficiency enhancement in the cyclotron autoresonance maser

The effect of the electron beam on the propagation of the radiation beam in a cyclotron autoresonance maser (CARM) is investigated. It is shown that the natural tendency of the radiation beam to spread diffractively can be overcome and the radiation beam can be focused because of its interaction with the electron beam. The radiation beam can be transported without defocusing following wave saturation only if the Doppler-shifted wave frequency is slightly larger than the gyrofrequency. Efficiencies in excess of 70% can be attained through the use of a tapered magnetic field. The effect of thermal spreads in the pitch angle and energy of the electrons on the guiding of the radiation beam and the efficiency of the CARM is investigated. The CARM is much more sensitive to spreads in pitch angle than to spreads in energy. If the spread in pitch angle becomes too large, typically a few percent, then the instability is quenched.

Mid-latitude spread- F structure

Spread- F has been observed at frequencies of 1.98, 3.84 and 5.80 MHz and multiple angles of arrival have been resolved using an HF radar near Brisbane (27°S, 153°E). The spreading of the ionogram trace has been shown to be due to a spread in angles of arrival of echoes, rather than any ‘vertical’ spreading. The reflection process appears to involve total specular reflection rather than scattering. The previously reported very strong bias for angles of arrival from the north-west at Brisbane is supported. The direction of movement of the reflection points is not radial and therefore, the structure cannot be purely frontal with purely linear movement, as is often supposed. The velocities are much less than for coexisting travelling ionospheric disturbances. The variations of angle of arrival, range and rate of change of range with frequency do not fit previously proposed ideas of the plasma distribution and an alternative is suggested in which the distortions of the isoionic surfaces resemble small, elongated, asymmetrical ‘hills’ or ‘dips’.

EPR of azurins from pseudomonas aeruginosa and alcaligenes denitrificans demonstrates pH-dependence of the copper-site geometry in pseudomonas aeruginosa protein

The X- and Q-band EPR spectra of Pseudomonas aeruginosa ( 63Cu)azurin and Alcaligenes denitrificans azurin have been measured at pH = 5.2 and 9.2, in the presence and absence of 40% glycerol. The EPR spectra of both proteins could properly be simulated by taking into account a spread in the tetrahedral angle of the copper site. The change in the EPR spectrum of Pseudomonas aeruginosa ( 63Cu)azurin that is observed upon an increase of the pH from 5.2 to 9.2 is consistent with a small decrease of the average tetrahedral angle from 61° to 60°. This geometrical change is consistent with the interpretation of earlier NMR and EXAFS observations. No pH effect is observed for Alcaligenes denitrificans azurin, in agreement with predictions based on crystallographic evidence. Glycerol has only a marginal effect on the appearance of the EPR spectra, and does not alleviate the “g-strain.”

Refined structure of glutathione reductase at 1.54 Å resolution

The crystal structure of human glutathione reductase has been established at 1.54 Å resolution using a restrained least-squares refinement method. Based on 77,690 independent reflections of better than 10 Å resolution, a final R-factor of 18.6% was obtained with a model obeying standard geometry within 0.025 Å in bond lengths and 2.4 ° in bond angles. The final 2 F o — F o electron density map allows for the distinction of carbon, nitrogen and oxygen atoms with temperature factors below about 25 Å 2. Apart from 461 amino acid residues and the prosthetic group FAD, the model contains 524 solvent molecules, about 118 of which can be considered an integral part of the enzyme. The largest solvent cluster is at the dimer interface and contains 104 interconnected solvent molecules, part of which are organized in a warped sheet-like structure. The main-chain dihedral angles are well-concentrated in the allowed regions of the Ramachandran plot. The spread of dihedral angles in β-pleated sheets is much larger than in α-helices and especially in α-helix cores, indicating the higher plasticity of β-structures. The analysis revealed a large amount of 3 10-helix. The side-chain conformations cluster at the staggered positions, and show well-defined preferences. Also, a mobility gradient is observed for side-chains. Non-polar and polar side-chains show average temperature factor increases per bond of 10% and 25%, respectively. A number of alternative conformations of internal side-chains, in particular serines and methionines, have been detected. The extended FAD molecule also shows a mobility gradient between the very rigid flavin ( 〈B〉 = 8.7 A ̊ 2 ) and the more mobile adenine ( 〈B〉 = 16.2 A ̊ 2 ). The entire active center is particularly well ordered, with temperature factors around 10 Å 2. The dimer interface consists of a rigid contact area, which is well conserved in the Escherichia coli enzyme, and a flexible area that is not. Altogether, the buried surfaces at the crystal contacts are half as large as at the dimer interface, but less specific. The refined structure shows clearly that there are no buried cations compensating the charge of the pyrophosphate moiety of FAD. The flavin deviates slightly from standard geometry, which is possibly caused by the polypeptide environment. In contrast to an earlier interpretation, atom N5 of the flavin can accommodate a proton, and it is conceivable that this proton proceeds to the redox-active disulfide. Cys58 of this disulfide may be activated in a manner similar to the “charge relay system” of serine proteases.

Computer-generated vitreous silica networks

A large fully-bonded periodic continuous random network model of vitreous silica is generated via a computer simulation. This is done by introducing the defects into a diamond lattice and then decorating with oxygen atoms. The model shows good agreement in radial distribution function with experiment although the spread in angles at the silicon atoms is too large.

Influence of thermal spread on space-charge limiting current

The effect of thermal spread in pitch angle on the limiting current of an electron beam is calculated. The downstream beam parameters are also evaluated. At the limiting current the downstream value of the pitch angle and the downstream thermal spread are found to be sensitive to the initial thermal spread.