Diffraction Picture Research Articles

Washing-out of the mean diffraction picture in the focal plane of a receiving lens, due to rain in a turbulent atmosphere

Washing-out of the mean diffraction picture in the focal plane of a receiving lens, due to rain in a turbulent atmosphere

Diffraction-broadening studies of textured materials

Reciprocal-space calculations are carried out with the aim of finding out where on an electron or X-ray diffraction picture one has to measure diffraction line broadening, so as to obtain reliable information about the apparent domain size (DS) and the microstrain (MS) in a textured material. It has been shown that in both cases there are geometrical limitations. They are greater for X-ray diffraction broadening studies, and if the texture axis is arbitrarily inclined, a readjustment of the texture axis to positions parallel to the X-ray beam, or parallel to the camera axis, is required. Deviation of the texture axis from an ideal orientation increases the number of diffraction spots requiring correct correlation of their diffraction broadening with the DS and MS of the material.

Structural deformation properties of organic fibers based on para-polyamides

On the basis of x-ray analytical data it has been established that the studied material has a liquid crystal type structure. In the direction along the fiber axis there is a further degree of ordering; in the radial direction further order is not observed. The dependence of the microdeformation on the deformation of the sample and external loading was studied from the change in the diffraction picture during loading. It was established that e μ is directly proportional to e. The closeness of e μ and e indicates a uniformity of the material in the structure relationship. It is suggested that the deformation mechanism of the given material is similar to the deformation mechanism of some materials of similar chemical structure which have been considered in the literature. It has been established that after loading the material retains a residual deformation both at the macro- and the microlevels. After thermal treatment the residual microdeformation disappears. It is assumed that e μ resid arises due to a partial fixation of the stretched state of the chains by strong intermolecular interaction. Small-angle diffraction provides little information with respect to studying the fracture and structure of the material. The experiments carried out may be lacking in some undetermined regimen of loading — in which during recording of the reflection (∼10 min) relaxation of the stress on the sample and creep occurs. However, qualitatively this cannot influence the results obtained.

Charge-density waves in NbSe3 at 145K: crystal structures, X-ray and electron diffraction studies

The crystal structure of NbSe3 has been refined from single crystal X-ray diffraction data. It has a monoclinic symmetry with lattice parameters: a=10.009 AA, b=3.4805 AA, c=15.629 AA, beta =109.47 AA, space group P21/m and six formulae per unit cell. The crystal structure of NbSe3 as determined at 100K shows that the 145K transition in the electrical resistivity is not accompanied by a structural distortion. Except for the decrease due to thermal contraction all interatomic distances are found to be the same. This is compatible with the proposed model of the charge-density-waves formation which has been put forward in order to explain the physical properties of the 145K transition. Electron diffraction pictures taken above and below the transition give direct evidence of the charge-density-waves formation at the transition. At 120K in agreement with Tsutsumi et al. (1977) the pictures contain superstructure spots at the (h,k+or-0.243, l) positions. Contrary to what has been reported by the same authors, the authors have found that at 3K above the transition the electron diffraction pictures do not contain the diffuse scattering streaks corresponding to the planes at k'=k+or-0.243, which would indicate fluctuations of the CDW above the transition.

Temperature Dependent Aggregation of Bacteriorhodopsin in Dipalmitoyl‐ and Dimyristoylphosphatidylcholine Vesicles

AbstractBacteriorhodopsin has been incorporated into large unilamellar vesicles of dipalmitoyl‐ and dimyristoylphosphatidylcholine. The state of aggregation of bacteriorhodopsin in these vesicles was investigated using X‐ray diffraction, freeze fracture electron microscopy, circular dichroism and rotational diffusion measurements.At temperatures below the lipid phase transition, the X‐ray diffraction results show that bacteriorhodopsin crystallizes into patches with the same hexagonal lattice as is observed in the purple membrane. This hexagonal lattice is clearly visible in the electron micrographs. Measurements of the rotational diffusion of the protein, using the decay of flash‐induced dichroism, show that the protein is completely immobilized. In agreement with these results the circular dichroism spectra exhibit the typical exciton coupling bands of the native lattice.Above the phase transition, the lattice disaggregates and the protein molecules are monomeric provided the lipid to protein ratio is sufficiently high. In the X‐ray diffraction pictures both the protein and the lipid long‐range order are absent. The freeze fracture electron micrographs show that the bacteriorhodopsin particles are randomly distributed in the plane of the lipid bilayer. In harmony with these observations rotational diffusion measurements demonstrate that the bacteriorhodopsin molecules are free to rotate about an axis normal to the plane of the membrane. The rotational correlation time shows a marked dependence on the lipid to protein ratio. The absence of exciton coupling bands in the circular dichroism spectra indicates that specific bacteriorhodopsin aggregates are absent.It seems probable that the mechanism of bacteriorhodopsin crystallisation involves the growth of crystalline lipid areas from which the protein is excluded.

Liquid Crystalline Structures in the Lecithin‐Cholate‐Water System Studied by NMR and X‐Ray Diffraction Methods

AbstractAlthough bile salts have several properties typical for a surfactant substance they show features that differ from those of ordinary amphiphiles. For instance, they do not aggregate in the cooperative manner and they never form lyotropic liquid crystalline phases. However, they have a large capacity to break lecithin bilayers by forming mixed micellar aggregates.The phase diagram for egg yolk lecithin‐sodium cholate‐water has been determined by Small and coworkers [1]. The extension and appearance of the different phases are similar to those in the phase diagrams for ordinary soap‐alcohol‐water systems. Concerning the structures on the molecular level of the different phases, suggestions have been given by Small, but no unambiguous experimental evidence for most of these structures have been reported.Here we present some different NMR studies in order to inquire into the molecular packing and order in the aggregates. Complementary x‐ray studies are also performed. 1H‐NMR shows that, in contrast to what is found for ordinary soap systems, spectra for lecithin‐cholate systems are essentially the same for the so called hexagonal (E) and lamellar phases at a given lecithin/cholate ratio. The order parameter calculated from 2H quadrupole splittings of the‐methyl groups of dilauroyl lecithin vary continuously between the E and lamellar phases, contrary to data obtained for soap systems. All these findings show that the molecular ordering is very similar in the two phases.It is also found that the molecular order decreases strongly with increasing cholate concentration.The x‐ray diffraction picture for the E‐phase shows reflexions that do not unambiguously fit the hexagonal structure.2H water quadrupole splittings of a between glass plates macroscopically oriented E‐phase sample showed that it orients uniaxially with at least a three‐fold symmetry around the symmetry axis.Lipid diffusion coefficients can be conveniently obtained with pulsed NMR for both cubic [2] and lamellar [3] phases. The diffusion coefficients obtained are of the same order of magnitude, leading to the conclusion that the molecular diffusion occur over macroscopical distances in the cubic phase [2,3]. Thus the cubic phase consists of continuous lipid aggregates. Preliminary diffusion data obtained for the E‐phase indicate that also this phase consists of continuous amphiphile aggregates. This is not compatible with the structure previously proposed, where micelles are stacked to form long rods.

A space-(-time) picture of hadronic diffraction

A new space (-time) mechanism for hadronic diffraction is presented. The geometrical concept of a three-dimensional long interaction region - a spheroid with the axes R↶ ≈ 1 fm and R ∥ ∞ √ s in the c.m.s. - plays an important role. The diffraction amplitude is obtained in closed analytic form exhibiting a diffraction peak at small t, an Orear-type behaviour e −Ap↶ at intermediate t and a power law behaviour s − N f( θ) at large angles.

On polarization effects in high-energy vector-meson photoproduction

In a recent paper (1) an analysis of vcetor-meson photoproduetion at high energies was presented in terms of a multiehannel impact-parameter formalism which provided a unified description of all diffractive photon-induced reactions. I t was argued there that a helieity-flip component of the asymptotic part of the amplitude, i.e. the pomeron, may well be present in the proposed unitarized multiple-scattering picture of diffraction and it was indeed subsequently found in the analysis of 7 ~ and p~ data. As is well known, ~0 photoproduction is an ideal testing ground for the properties of the Pomeranchuk contribution since no other Regge exchanges are supposed to contr ibute there (2). I t was shown that for the c~~ amplitude (and for the i~ ~ amplitude) there is a substantial amount of helicity flip at the proton vertex (*). Of course, this model-dependent result has to be confirmed by direct measurement of the relevant density matrix elements. Unfortunately, the present data cannot provide information about this point because of lack of appropriate data on polarization experiments. One is therefore forced to use the available differential cross-section data in order to get information on this particular helicity amplitude with flip at the nucleon-~nucleon transition. I t seems therefore worth-while to discuss the possible experiments with polarized beams and/or polarized target which could directly test our rather indirect finding of a violation of SCIIC (s-channel helicity conservation) (a) for the recoiling proton in ~o (po) photoproduction. In view of the recent progress in preparing polarized targets possessing polarizations up to (70--80)~o it is feasible to carry out such experiments in the near future. Let us represent the state of polarization of the incoming nucleon and of the photon by the spin density matrices (4.5)

On some experiments leading to a revision of the foundations of quantum mechanics

When a homogeneous electron beam passes through a hole in a screen, according to quantum mechanics a well-known diffraction picture is obtained. It does not depend on the density ϱ of the electrons, because Schrödinger’s equation is linear. But the experiment contradicts this deduction, because actually the cross-section of the beam dilatates behind the screen for large ϱ. According to the new viewpoint on quantum mechanics proposed by the author, a generalized nonlinear Schrödinger equation is obtained. From this it follows both a dilatation of the electron beam and a change in de Broglie’s wavelength, depending on ϱ. The attempt to check these effects may turn into a crucial experiment.

Elastic scattering from the two-component picture

The momentum transfer dependence of high-energy elastic scattering is discussed in the context of the multiperipheral + diffraction picture. Both a “naive” and an “absorbed” model are considered. Fits are made to the impact parameter dependence of the pp elastic amplitude. It is essential in this picture that the diffractive component be peripheral; in the fits it has a zero at | t| between 0.2 and 0.4. The origin of the small t-break in this picture is discussed, and contrasted with its explanation in similar fits.

Holography without fringes in the electron microscope.

I REPORT here the successful solution of the phase problem for a negatively stained catalase crystal using diffraction and image plane pictures produced in an electron microscope. The results show the complete wave function, phase as well as amplitude maps, in-the microscope image plane. The computational method employed in achieving the solution has been described by Gerchberg and Saxton1 who demonstrated the technique on general mathematical functions; the method was later successfully applied to mathematical models of magnetic thin films2. I have now applied the method to a real biological crystal in the electron microscope.

Structure of insect fibrillar flight muscle in the presence and absence of ATP

Low-angle X-ray diffraction pictures were taken of Lethocerus flight muscle in one or other of its two inactive states, relaxation and rigor. They showed more detail than previous pictures and the parameters of the actin and myosin helices could be deduced from the spacings of the observed layer lines. Subunits consisting of actin monomers and of myosin heads were arranged on helices so as to conform to these parameters, and the diffraction patterns from these models were calculated. When these model systems resembled the structures observed in the electron microscope, the calculated diffraction patterns had layer Unes similar in radial distribution and relative intensity to those observed, with certain exceptions. The fit was quite critical, in that variation of the size and orientation of the subunits affected it considerably. The results support the idea that in rigor (i.e. without ATP) all of the myosin heads attach to actin monomers in a regular angled configuration. In contrast, on relaxation (i.e. on addition of ATP but no Ca 2+) most or all of the myosin heads detach from the actin and are arranged with a specific symmetry around the myosin filament. It is not necessary, however, to assume that they change shape or move far from the actin filament in this process. Features of the X-ray diffraction pattern which remained unaccounted for by this model can be explained on the basis of an arrangement of actin helices on a further helix around a thick filament. Types of extended lattices containing actin helices in statistical axial or azimuthal positions are discussed.

Relations between the structure of crystalline and amorphous CdGe xAs 2

A detailed discussion of the CdAs 2 structure makes it possible to present a model of the crystalline CdAs 2-type-to-chalcopyrite-type structure transformation. Unoccupied A- and B-sites are introduced into the CdAs 2 lattice. Macroscopic density measurements on the amorphous samples of the CdGe x As 2 system are interpreted by a mechanism of gradual occupation of these A- and B-sites with Ge atoms. A rough interpretation of diffraction pictures taken on amorphous samples with x = 0.1 and 1.1 points to a close similarity of these amorphous samples with the CdAs 2 nearest neighbours structure.

The lifetimes of positrons in oxides of arsenic and antimony

The lifetimes of positrons in crystalline powders of dimeric oxides of arsenic and antimony, and in trioxymethylene and polyoxymethylene are reported. In As4O6 and polyoxymethylene a longer componentτ4 is observed in addition to the usually observedτ2 component. The variation ofτ4 with temperature and pressure is studied in As4O6. On the basis of these results and the X-ray diffraction pictures, it is suggested that ortho-positronium atoms quenched in intercrystallite regions in As4O6 gives rise toτ4.

Crystallographic studies of a human immunoglobulin.

The first X-ray diffraction pictures of an intact immunoglobulin protein have been obtained.

Diffraction Scattering and Vector-Meson Resonances

It is shown that, under reasonable assumptions about inelasticity and asymptotic behavior, the usual diffraction picture combined with the N/D approach to π-π scattering leads to a singular integral equation. The authors' formalism is then used to show that, in conformity with the nearby-singularities philosophy, a constant left-hand discontinuity is by itself incapable of producing resonances in the GeV region. Next, a model for the creation of vector resonances, which combines a long-range force (defined by exchange of a cutoff vector meson) plus a short-range force (compatible with diffraction requirements), is introduced. The effect of the short-range force on the self-consistent (bootstrap) solutions is investigated in an approximate scheme. For a self-consistent solution with the correct ρ-meson mass, which is found to exist, inclusion of the short-range force is shown to decrease the self-consistent width by a factor of 2, which is nevertheless still greater than the experimental value.

Electron diffraction at crystal surfaces: II. The double-diffraction picture

A highly simplified theoretical picture of electron diffraction at crystal surfaces previously proposed on intuitive grounds by Bauer and others, is here developed systematically on the basis of the dynamical theory given in I. It is shown that the application of nondegenerate second-order perturbation theory to the matrix eigenvalue problem in I leads to an expression for the diffraction amplitude, b, of the form b=b (1)+b (2) , where b (1) denotes the contribution from single-diffraction processes at different atom layers, summed over all atom layers and b (2) denotes the contribution from sequences of double-diffraction processes at different atom layers, summed over all pairs of atom layers. The term double-diffraction as used here includes processes involving an evanescent intermediate wave. The term b (1) represents a modified kinematical description. The description including both b (1) and b (2) is called the “double-diffraction” picture. Explicit expressions are given for b (1) and b (2), for a model crystal with a one-layer selvedge. The physical meaning and limits of validity of the double-diffraction picture are discussed. Applications to the following three topics in the field of low-energy electron diffraction are commented on briefly: secondary peaks, fractional-order beams and satellite beams.

Evaluation of the degree of three-dimensional ordering in the structure of pyrographite

1. An examination is made of the x-ray diffraction methods of determining the degree of three-dimensionality of structure (degree of graphitization) of carbon-graphite materials as typical examples of lamellar materials with layer stacking faults. It is shown that the type of x-ray diffraction picture changes substantially on transition from the three-dimensional to the two-dimensional graphite structure. The existing methods of determining the degree of graphitization are based on analysis of the angular position of the lines 00l and the intensity of the lines hkl. 2. A semiquantitative method is proposed for evaluating the degree of graphitization on the basis of analysis of the relative intensity of the lines hk0 (Px method). The crystallographic nature of the redistribution of hkO line intensity with change in the degree of graphitization is examined. 3. The Px method was applied to the analysis of the degree of graphitization of thin low-temperature pyrographite films. Correlation between Px values and the asymmetry of the lines hk in electron diffraction pictures was established. 4. It is demonstrated that the Px value for high-temperature pyrographite, determined by the electron diffraction method, is in good agreement with the Px and Pd values calculated from x-ray diffraction data.

Niveaux collectifs dans le 32S

Angular distributions of elastically and inelastically scattered alpha particles on 32S are presented for an incident energy of 23.8±0.2 MeV. the distributions corresponding to the ground state and first excited state are in agreement with the simple picture of diffraction scattering. It is shown that the scattering behaviour and electromagnetic properties of the 3.78 MeV level and the 4.29–4.47 levels (not resolved) are in disagreement with a vibrational description.

Morphology of solution-crystallized polypropylene

Results are presented of the structures obtained upon crystallization of polypropylene from dilute solutions in α-chloronaphthalene. It is shown that under proper conditions of concentration and crystallization, isotactic polypropylene can be caused to grow in the form of lath-shaped single-crystal lamellae. Electron diffraction pictures of these lamellae indicate a monoclinic crystal structure. Crystals have been grown both from whole polymer and from sharp molecular weight fractions. When crystallization from dilute solutions (0.4–0.001 wt.-%) is allowed to take place under ambient cooling conditions, the crystals formed are not well defined, but tend to be more ordered if fractions are used. When fractions are crystallized under constant temperature conditions (from 90 to 115°C.) the resulting structure formed tends to be a close-packed interwoven array of individual elements. However, if whole polymer is used the aggregate has a more open weave structure and, upon deposition upon a carbon substrate, the elements making up the quadrites tend to collapse at the edges. Electron transmission and diffraction pictures show these elements to be single lamellar crystals of polypropylene with well-defined edges and with thicknesses of the order of 125–150 A. Fracture of crystals by ultrasonic vibration leads to the presence of fibrils between the separated parts when the break is at right angles to the a direction, but to no fibrils when the break is parallel to this direction. These results suggest that the polypropylene chains are folded, and that the fold planes are parallel to the a crystallographic axis