Laue case reflection curves of deformed crystals

The stationary phase method is used for the calculation of the reflection curves in the case of elastically bent crystals. This method, based on the Takagi s equations, enables us to estimate the widths of these curves and to compare them with the experimentally measured ones.     

Diffraction reflection of x-ray radiation with a two-dimensionally bounded wavefront from perfect crystals

The problem of dynamical diffraction of x-ray radiation with a two-dimensionally bounded wavefront is solved in the Bragg and Laue geometries in a crystal with an arbitrary thickness and an arbitrary reflection asymmetry parameter. An analysis of the wavefront deformation during the diffraction and subsequent propagation is carried out. It is shown that the most favorable conditions for the reflected beam to retain its shape are accomplished in a crystal whose thickness is less than the extinction depth.     

Features of the excitonic optical reflection curves of GaAs crystals

The excitonic optical reflection curves are investigated for GaAs crystals subjected to a surface-sensitive electron bombardment. The observed features of the curves can be explained on a qualitative level by using an extremely simple square-well approximation of the subsurface excitonic potential. Fiz. Tverd. Tela (St. Petersburg) 39, 610–612 (April 1997)     

Electronic-structure calculations for complex crystals

Summary We discuss some new ideas for improving first-principle muffin-tin orbital-type electronic-structure calculations for complex crystals such as ternary and multinary compounds. The new ideas include construction of generalized muffin-tin orbitals and energy-dependent double MTO basis sets, cellular decomposition based on strongly overlapping atomic spheres and evaluation of matrix elements in the residual interstitial region by extrapolation. Supported in part by the Office of Naval Research, Contract Number N00014-79-C-0814.     

Simplified formulas for non-normal reflection from monoclinic crystals

Based on Berreman's 4 × 4 matrix formalism [W. Berreman, J. Opt. Soc. Am. 62, 502 (1972)] we derive simplified expressions for the reflection formulas from the a-c crystal surface for non-normal incidence. The computational effort to apply the formulas was found to be only insignificantly higher than those for normal incidence, but considerably lower than for the general expressions. Consequently, the simplified formulas can be readily applied for dispersion analysis of spectra gained by spectroscopic ellipsometry or attenuated total reflection.     

Pseudopotential calculations for hydrogen centers in ionic crystals

Pseudopotential cluster calculations have been performed for hydrogen centers H⁰_i, H⁰_a, H⁰_c in XF and XCl ionic crystals (X = Li, Na, K, Rb). Results are given for interatomic potentials, for frequencies of localized vibration modes and for activation energies for diffusion of the hydrogen centers.     

Building the pyramids: perfect bubble crystals

Abstract

The use of appropriate flat-sided pyramidal containers to grow ordered foams allows single crystals to be formed. In the case of face-centred cubic crystals, these have been prepared with up to 500 bubbles. Strained and deliberately defective crystals can also be grown. The growth of simple cubic and body-centred cubic crystals is limited by instability; preliminary results are presented for these, as well as ordered bidisperse foams.     

Parametric reflection phenomenon in quadratic uniaxial crystals with birefringence

Effects of parametric refraction and reflection of optical beams in uniaxial crystals have been considered with allowance for birefringence. The equation of the signal wave trajectory, taking into account the medium anisotropy, has been obtained and analyzed. Pecularities of these effects for different types of three-frequency interaction of ordinary and extraordinary waves are shown. In the presence of birefringence, the effective mismatch sign can change and consequently the cascade nonlinearity can become focusing; in this case, the effect of reflection disappears.     

Size effects on polaritons in thin crystals with perfect interference spectra

Size effects on polaritons in thin plane parallel CdS and CdSe crystals are discussed. For samples not thinner than 0.1 μm the measurements of transmittance and reflectance show the size quantization of the mechanical exciton dispersion and a shift of the resonance frequency of the exciton to the high-energy side with decreasing thickness. However no evidence for the alteration of the oscillator strength or other parameters has been obtained. This conclusion is in conflict with the recently published data taken on CdS samples. The stability of the polariton dispersion in our case is supported by the theoretical study of reflectance ignoring the effects of the finiteness of the exciton radius.     

Direct determination of X-Ray reflection phases for noncentrosymmetric crystals

An experimental method is described for the phase determination of x-ray reflections from noncentrosymmetric crystals. This method results from considerations on the phase dependence of line profiles in three-beam reflection-type diffractions for wavelength above and below a critical absorption edge _E of the heaviest constituent atom in the crystals. A relationS _p =S _L ·S _R for phase determination is derived theoretically and verified experimentally for< _E ,S _p being the sign of the sine of invariant phase,S _L the sign defined from the line profile, andS _R determined by the rotation of the crystal lattice.     

Dynamic reflection holograms in CdF2 crystals with bistable centers

The diffraction efficiency and the recording and relaxation times of dynamic reflection holograms, recorded in CdF₂ crystals with bistable centers are studied experimentally in the temperature range 20–100°C. In the model experiments which measured the quality of the wave reflected from the hologram, the dynamic wavefront distortions are demonstrated to be efficiently compensated using a holographic corrector based on these crystals. CdF₂ crystals with bistable centers are likely to be useful in solving problems of correction of laser light wavefront and image correction in observation telescopes with nonideal primary mirrors.     

Features of X-ray diffraction in thick perfect crystals with deformed subsurface layers

Diffraction of X-rays in thick perfect crystals with deformed subsurface layers has been investigated. It is shown that in deformed layers kinematic scattering occurs, while in the bulk of perfect part dynamic scattering takes place. It is revealed that in the beams diffracted on surface defects, an essential role plays the second harmonic of the employed characteristic MoK_α1 radiation.     

Denisiuk-type reflection holography display with sillenite crystals for imaging and interferometry of small objects

A novel setup for imaging and interferometry through reflection holography with Bi₁₂TiO₂₀(BTO) sillenite photorefractive crystals is proposed. A variation of the lensless Denisiuk arrangement was developed resulting in a compact, robust and simple interferometer. A red He-Ne laser was used as light source and the holographic recording occurred by diffusion with the grating vector parallel to the crystal [0 0 1]-axis. In order to enhance the holographic image quality and reduce noise a polarizing beam splitter (PBS) was positioned at the BTO input and the crystal was tilted around the [0 0 1]-axis. This enabled the orthogonally polarized transmission and diffracted beams to be separated by the PBS, providing the holographic image only. The possibility of performing deformation and strain analysis as well as vibration measurement of small objects was demonstrated.     

The preparation of “perfect” dendritic germanium crystals

Perfect germanium crystals with the required resistivity and small dislocation density are reproducibly prepared in the Popov Research Institute of Radiocommunications. Perfect dendrites are suitable for use, for example, in the preparation of alloy diffused transistors. The shape, pulling apparatus and actual preparation of perfect germanium dendrites are described. Some of the parameters influencing the growth of a perfect dendrite are analyzed and the optimum conditions for its growth are determined.     

Absorbed dose calculations for macromolecular crystals: improvements to RADDOSE

Radiation damage is an unwelcome and unavoidable aspect of macromolecular crystallography. In order to quantify the extent of X‐ray‐induced changes, knowledge of the dose (absorbed energy per unit mass) is necessary since it is the obvious metric against which to plot variables such as diffraction intensity loss and B factors. Significant improvements to the program RADDOSE for accurately calculating the dose absorbed by macromolecular crystals are presented here. Specifically, the probability of energy loss through the escape of fluorescent photons from de‐excitation of an atom following photoelectric absorption is now included. For lighter elements, both the probability of fluorescence and of its subsequent escape from the crystal are negligible, but for heavier atoms the chance of fluorescence becomes significant (e.g. 30% as opposed to Auger electron decay from a K‐shell excited iron atom), and this has the effect of reducing the absorbed dose. The effects of this phenomenon on dose calculations are presented for examples of crystals of an iron‐containing protein, 2‐selenomethionine proteins, a uranium derivatised protein, and for a nucleic acid sample. For instance, the inclusion of fluorescent escape results in up to a 27% decrease in the calculated absorbed dose for a typical selenomethionine protein crystal irradiated at the selenium K‐edge.     

The use of perfect crystals in high-resolution X-ray spectroscopy

The possibility of improving the resolution and angular dispersion in the X-ray wavelength region through the use of asymmetric crystals in parallel beams is demonstrated.