Showing papers on "Resolution (electron density) published in 1987"

Magnetic imaging by ‘‘force microscopy’’ with 1000 Å resolution

Abstract: We describe a new method for imaging magnetic fields with 1000 A resolution. The technique is based on using a force microscope to measure the magnetic force between a magnetized tip and the scanned surface. The method shows promise for the high‐resolution mapping of both static and dynamic magnetic fields.

X-ray diffraction computed tomography

Abstract: Coherent scattering of x-ray photons leads to the phenomenon of x-ray diffraction, which is widely used for determining atomic structure in materials science. A technique [x-ray diffraction computed tomography (CT)] is described, analogous to conventional CT, in which the x-ray diffraction properties of a stack of two-dimensional object sections may be imaged. The technique has been investigated using a first generation (single pencil beam) CT scanner to measure small angle coherent scatter, in addition to the customary transmitted radiation. Diffraction data from a standard CT performance phantom obtained with this new technique and with an x-ray diffractometer are compared. The agreement is satisfactory bearing in mind the poor momentum resolution of our apparatus. The dose and sensitivity of x-ray diffraction CT are compared with those of conventional transmission CT. Diffraction patterns of some biological tissues and plastics presented in a companion paper indicate the potential of x-ray diffraction CT for tissue discrimination and material characterization. Finally, possibilities for refinement of the technique by improving the momentum resolution are discussed.

Dynamic reflection interference contrast (RIC-) microscopy : a new method to study surface excitations of cells and to measure membrane bending elastic moduli

Abstract: 2014 A new method, the dynamic reflection interference contrast (RIC) microscopy is presented by which thermally excitated surface undulations of erythrocyte plasma membranes 2014 the cell flickering 2014 can be evaluated to an amplitude resolution of ~ 50 nm and a wavelength resolution of 0.5 03BCm. The Newtonian interference pattern formed by the interference of light reflected from the cell surface and from the cover glass (to which the cells are slightly fixed) is analysed by a home made fast image processing system. Two evaluation procedures are proposed : firstly the direct reconstruction of the momentaneous surface profile by retransformation of the RIC interference pattern and secondly the Fourier analysis of the interference pattern. In the first case the excitation relief is obtained by subtraction of two momentaneous surface profiles and it is best suited in the long wavelength regime. In the second case the spatial frequency spectrum of the flickering is determined by Fourier transformation of the RIC interference pattern. This technique is reliable in the short wavelength regime and is used here to determine the bending elastic modulus, Kc, in a wave length domain between 0.5 and 1 03BCm. This simultaneous determination of many Kc-values greatly enhances the accuracy of the measurement. This procedure is also suited to test whether the flickering obeys the equipartition law. The elastic constants of normal discocytes, of cup-shaped stomatocytes and of echinocytes are compared. The values of the first two classes are about equal : Kc = 3.4 ± 0.8 x 10-20 Nm and agree well with values reported previously [1]. The last cell shape exhibits a substantial higher stiffness Kc = 13 ± 2 10-20Nm. An outstanding advantage of the dynamic RIC-microscopy is that it allows to measure absolute values of the displacement of the membrane facing the coverslide whereas the conventional flicker spectroscopy [1, 8] can only detect fluctuations of the cell thickness. A second advantage is that it can also be applied to erythrocyte ghosts or to other transparent cells. J. Physique 48 (1987) 2139-2151 DTCEMBRE 1987, Classification Physics Abstracts 87.20 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480120213900

The Response of Biological Macromolecules and Supramolecular Structures to the Physics of Specimen Cryopreparation

Abstract: In the preceding chapter we have learned about the problems of freezing and have seen that one of the major problems in cryotechniques is the formation of ice crystals (see Bachmann and Mayer, Chap. 1, this Vol.). Due to their size, they disturb the fine structure of cells (see also Sitte et al., Chap. 4, this Vol.; Moor, Chap. 8, this Vol.). In the present chapter, we consider the problems which we face when we are able to work in vitreous ice, i.e. what happens with biological macromolecules and supramolecular structures. In these considerations we distinguish two levels of resolution: at the highest resolution (1–10nm), we consider conformational changes of macromolecules, particularly of proteins. This resolution is, in general, only achieved with procedures that allow for averaging from identical subunits. At the other level, we consider rearrangements and small aggregations of macromolecules due to the specimen preparation. This is the resolution of 10–20 nm achieved normally with the current methods of preparation of biological material.

High-resolution x-ray diffraction studies of InGaAs(P)/InP superlattices grown by gas-source molecular-beam epitaxy

Abstract: High‐resolution x‐ray diffraction studies have been carried out to determine the structural perfection and periodicity for a number of high‐quality InGaAs/InP superlattices and one InGaAsP/InP superlattice grown by gas‐source molecular‐beam epitaxy. For comparison, high‐resolution diffraction both with a three‐crystal geometry and with a four‐crystal monochromator was used along with conventional double‐crystal x‐ray diffractometry. The best resolution in the x‐ray satellite patterns was obtained with the four‐crystal monochromator, providing a resolution of one molecular layer in the periodicity of the superlattice. The presence of sharp satellite reflections in the x‐ray diffraction profiles demonstrate smooth interfaces with well‐defined modulated structures which could be derived from a kinematical diffraction step model. For some superlattices, excellent agreement between the step model and the measurements is obtained when the model assumes that each period consists only of the well and the barrier ...

Technique for shaping scanning tunneling microscope tips

Abstract: Two innovations have been applied to improve a method developed earlier for the production of field‐ion tips. The new technique produces sharper, smaller tips with low‐aspect ratio shanks to fulfill the specific needs of scanning tunneling microscopy.

Confocal microscopy comes of age

Abstract: Scanning confocal optical microscopy offers increased resolution and superior rejection of out-of-focus signals for epifluorescence imaging of biological material.

Super-resolution in confocal scanning microscopy

Abstract: In a previous paper, (Bertero et al., Opt. Acta. vol.3., p.923 (1984)), it was shown that the resolution of the type-II confocal scanning microscope may be improved by recording the full image and by inverting the data. In this paper, the authors propose a numerical method which can be easily implemented and, in principle, applied to the practical problem. They show that by using a rather small number of data points on the image plane, it is possible to obtain the improvement in resolution (by a factor of two) predicted in their previous analysis.

Residual electron density at the M2 site in C2/c clinopyroxenes: Relationships with bulk chemistry and sub-solidus evolution

Abstract: The systematic study of both natural and synthetic clinopyroxenes often indicates the presence in the difference Fourier map of a maximum of residual density of up to 0.8 electrons, here labelledM2′, close to theM2 site along the diad axis, defining a square pyramid co-ordination polyhedron. To investigate the nature and the crystalchemical implications of this feature, a limited but representative set of clinopyroxenes of volcanic, metamorphic and synthetic origin has been investigated by X-ray structure refinement (at 0.7 and 0.4 A resolution), by microprobe analysis and by transmission electron microscopy. The most important results are: a) at increasing resolution, the height of theM2′ peak increases while its co-ordinates move towardM2; b) as (Ca + Na) content approaches 1.0 atom per formula unit,M2′ vanishes; c)M2′ has been found in clinopyroxenes which show differing incipient exsolution microstructures, from spinodal decomposition to non-periodic fluctuations, as well as in homogeneous specimens. The presence ofM2′ is interpreted in terms of the simultaneous coexistence in the crystals of two different structural models, approximately diopside and clinoenstatite. An accurate evaluation of the totalM2 + M2′ site occupancy is strongly suggested in XREF work, particularly when thermodynamic and kinetic considerations have to be obtained with accurate determinations of site occupancy factors as a starting point.

Calibration of the photoluminescence technique for measuring B, P and Al concentrations in Si in the range 1012 to 1015 cm-3 using Fourier transform spectroscopy

Abstract: Calibrations are reported for measuring boron, phosphorus and aluminium concentrations in silicon by comparing the intensities of bound-exciton and free-exciton luminescence features. Particular attention has been directed towards investigating the effects of excitation density and instrumental resolution. The luminescence spectra have been corrected for the system response so that the calibrations are independent of the equipment used and are hence transferable. Fourier transform rather than dispersive spectroscopy has been employed to measure the luminescence spectra. Since Fourier transform photoluminescence spectroscopy is a relatively new technique, it is described in some detail.

The structure determination of a common cold virus, human rhinovirus 14

Abstract: The methods used to solve the structure of human rhinovirus 14 at 3.0 A resolution are described in detail. The crystals are cubic, space group P213, a = 445.1 A with 20-fold non-crystallographic redundancy and with approximately 55% solvent and RNA content. The data used to solve the structure were collected at the Cornell High Energy Synchrotron Source (CHESS) using oscillation photography. Most of the computations were performed on Purdue University's Cyber 205 supercomputer. Two heavy-atom derivative data sets from crystals soaked in 1 and 5 mM KAu(CN)2 were used to provide isomorphous phasing to 4 A resolution, although it was subsequently shown that phases beyond 5 A resolution were random. The phases were refined at 5 A resolution by five cycles of real-space molecular replacement. Phase extension from 5 to 3 A was then performed using 60 cycles of real-space molecular replacement while extending the resolution in steps of three reciprocal-lattice points at a time once every three cycles. The 3.5 A skew-averaged map was easily interpreted and showed 811 of the 855 amino acids in the four distinct viral polypeptide chains. A complete atomic model has been built using FRODO on an Evans & Sutherland PS300 graphics system with respect to the 3.08 A resolution electron density map. The roles of the non-crystallographic symmetry, solvent content, errors in amplitudes, orientation and translation in the molecular replacement process are discussed.

Picosecond photoelectron scanning electron microscope for noncontact testing of integrated circuits

Abstract: A scanning electron microscope which uses an ultrashort pulsed laser/photocathode combination as an electron source produces electron pulses of order 1 ps in duration at a 100‐MHz repetition rate and with a peak brightness of 3×108 A/cm2 steradian at 1.8 keV. By using this instrument in the voltage contrast mode, without contact with the samples, we have been able to measure electrical pulses propagating on coplanar transmission lines with, simultaneously, a temporal resolution of 5 ps, a voltage resolution of 3 mV/(Hz)1/2, and a spatial resolution of 0.1 μm. These measurements are achieved with extraction fields above the sample of about 1 kV/mm.

Proposal for a Phase Contrast X-ray Microscope

Abstract: In x-ray microscopy experiments performed up to now the contrast is dominated by photoelectric absorption. The resulting radiation dose, though much less than in electron microscopy, limits fundamentally the resolution which can be obtained when investigating biological materials in the natural (intact wet and unstained) or living state, Sayre et al. (1977). Investigations to decouple resolution and radiation dose leads to a proposal for a phase contrast x-ray microscope. This work is based on experimental results on phase zone plates, Hilkenbach and Thieme (1986), and on the atomic scattering factors published by Henke (1981).

High resolution and the accurate measurement of intensities

Abstract: Absolute intensities in several vibration rotation bands of CO2 and in the pure rotation spectrum of CO have been measured. The results have been used to assess the precision and accuracy which such measurements can attain when made with a high resolution Fourier transform spectrometer.

Three-dimensional structure of deoxycholate-treated purple membrane at 6 Å resolution and molecular averaging of three crystal forms of bacteriorhodopsin

Abstract: The three-dimensional structure of the deoxycholate-treated form of purple membrane has been determined to a resolution of about 6 A. Using low temperature electron diffraction data, room temperature electron microscope images and improved methods of data analysis, higher resolution has been reached than was obtained using native membranes of the same size. Statistical analysis of the data shows that the new map is considerably better than earlier maps. The map indicates the probable sites for the lipid molecules that remain in the deoxycholate-treated membranes; some of these sites differ from those suggested by the projection map of Glaeser et al. (1985). Comparison of the bacteriorhodopsin structures now determined independently from three crystal forms shows that the monomer structure is independent of the detailed contacts with lipid molecules. The average of the three structures gives a picture with very little noise showing seven similar rod-like features which are clearly best interpreted as α-helices; there is no indication that part of the structure is β-sheet as suggested by Jap et al. (1983). Phases from the averaged structure at 6 A resolution will enable better refinement of the parameters that will be required in the analysis of higher resolution images from tilted specimens needed to extend the projection map at 3.5 A resolution (Henderson et al. 1986) to produce a three-dimensional atomic resolution map.

High resolution interferometric Fourier transform infrared absorption spectroscopy in a supersonic free jet expansion

Abstract: The absorption spectrum of CHF3 in a free jet expansion has been recorded at a resolution of 0·004 cm-1 in the 8–9 μm region. The line width is found to be essentially Doppler limited. An approximate rotational temperature of 40K has been determined from the spectrum. Relative intensities and transition moments for the interacting bands are discussed.

Resolution function of an X-ray triple-crystal diffractometer

Abstract: The resolution function of an X-ray triple-crystal diffractometer is calculated on the assumption that the resolution is controlled by the properties of the monochromating and analysing crystals. The expressions are then evaluated when these crystals have either a mosaic structure or when they are perfect flat crystals with a reflectivity controlled by the Darwin width. Within the Gaussian approximation for the Darwin curve, simple expressions for the resolution are then obtained both for a conventional X-ray source and for an X-ray synchrotron source, although the expressions differ in detail. The expressions are used to discuss the intensity obtained when a triple-crystal diffractometer is used to measure the integrated Bragg reflection intensity, the intensity associated with rods in reciprocal space and the intensity of diffuse scattering.