Formation of artificial opals viewed in situ by X-ray grazing insidence diffraction

Formation of artificial opal films by a vertical deposition method was in situ studied using the grazing incidence X-ray diffraction technique. Monodisperse spherical polymethyl methacrylate particles (200 nm in diameter) were deposited on the polished Si substrate from an aqueous suspension. The ordering of particles on a fixed area of the substrate located in turn in the bulk suspension under the meniscus and in the air was continuously monitored by the X-ray scattering upon moving the meniscus down owing to evaporation of the solvent. The triple air-liquid-solid phase boundary, i.e. the top line of the meniscus, is identified as the most probable location of the crystallization process. The analysis of observed Bragg reflections and the particle form-factor indicates that the obtained artificial opal-like structures are composed of the spheres arranged in a close-packed hexagonal layers parallel to the substrate. The characteristic correlation length along the normal to the substrate of 550 ± 100 nm is obtained from the half full width of Bragg peaks.     

Positron annihilation in electron-irradiated n-type gap crystals

Abstract

The defects in n-GaP crystals irradiated by 2.3 MeV electrons up to 1 × 10¹⁹ cm^?2 at RT were studied by means of positron annihilation (angular correlation) and electrical property measurements. It was found that positrons are trapped in some radiation-induced vacancy-type defects (acceptors) but that the effect saturates at high electron fluences (D1 × 10¹⁸ cm^?2). The trapping rate in irradiated samples increases with temperature in the range 77–300 K. Post-irradiation isochronal annealing reveals the positron traps clustering at about 200–280°C. All positron sensitive radiation-induced defects disappear upon annealing up to 500°C.     

Comparing normal modes across different models and scales: Hessian reduction versus coarse‐graining

Dimension reduction is often necessary when attempting to reach longer length and time scales in molecular simulations. It is realized by constraining degrees of freedom or by coarse‐graining the system. When evaluating the accuracy of a dimensional reduction, there is a practical challenge: the models yield vectors with different lengths, making a comparison by calculating their dot product impossible. This article investigates mapping procedures for normal mode analysis. We first review a horizontal mapping procedure for the reduced Hessian techniques, which projects out degrees of freedom. We then design a vertical mapping procedure for the “implosion” of the all‐atom (AA) Hessian to a coarse‐grained scale that is based upon vibrational subsystem analysis. This latter method derives both effective force constants and an effective kinetic tensor. Next, a series of metrics is presented for comparison across different scales, where special attention is given to proper mass‐weighting. The dimension‐dependent metrics, which require prior mapping for proper evaluation, are frequencies, overlap of normal mode vectors, probability similarity, Hessian similarity, collectivity of modes, and thermal fluctuations. The dimension‐independent metrics are shape derivatives, elastic modulus, vibrational free energy differences, heat capacity, and projection on a predefined basis set. The power of these metrics to distinguish between reasonable and unreasonable models is tested on a toy alpha helix system and a globular protein; both are represented at several scales: the AA scale, a Gō‐like model, a canonical elastic network model, and a network model with intentionally unphysical force constants. Published 2012 Wiley Periodicals, Inc.     

Geometric limits of knot complements, II: graphs determined by their complements

We prove that there are compact submanifolds of the 3-sphere whose interiors are not homeomorphic to any geometric limit of hyperbolic knot complements.     

In Situ Formation of Gold Nanoparticles within a Polymer Particle and Their Catalytic Activities in Various Chemical Reactions

Composite materials consisting of nanoscale gold particles and protective polymer shells were designed and tested as catalysts in various chemical reactions. Initially, the systematic incorporation of multiple gold nanoparticles into a poly(N-isopropylacrylamide) particle was achieved by an in situ method under light irradiation. The degree of gold nanoparticle loading, along with the structural and morphological properties, was examined as a function of the amount of initial gold ions and reducing agent. As these gold nanoparticles were physically-embedded within the polymer particle in the absence of strong interfacial interactions between the gold nanoparticles and polymer matrix, the readily-accessible surface of the gold nanoparticles with a highly increased stability allowed for their use as recyclable catalysts in oxidation, reduction, and coupling reactions. Overall, the ability to integrate catalytically-active metal nanoparticles within polymer particles in situ allows for designing novel composite materials for multi-purpose catalytic systems.     

Realization of a Given Relative Motion of two Rigid Bodies by a Two-Armed Robot

The formulation of a new task of realizing a given relative motion of two rigid bodies by means of a robot having two manipulators is given. A method for constructing the movements of two-armed robots is proposed, which consists of solving the direct and inverse problems of manipulator kinematics, taking into account additional conditions for eliminating the dangerous rapprochement of links.Equations of relationships of parameters of relative motion of the working links of the manipulators are obtained with the relative movements of their adjacent links assumed for control functions. Expressions for control functions of manipulators are obtained for the realization of a given relative motion of the general form of two rigid bodies, and also for plane motion.Pointing to the possibility of crossing the working links under the action ofmanipulators in one work area, and obtained the relationships that allow these intersections to be avoided.     

Basic results of investigations of scission neutrons in nuclear fission at low excitation energies

To estimate the main characteristics of neutrons emitted shortly before the scission of a fissioning nucleus, various experiments sensitive to the presence of these scission neutrons in thermal-neutron-induced fission of ²³⁵U and spontaneous fission of ²⁵²Cf were performed. The results of the experiments were analyzed within theoretical calculations allowing for various possible neutron-emission mechanisms, including the possibility of the emergence of neutrons from the scission of a nucleus.     

Radiation of channeled 800 MeV electrons in diamond crystals

Measured data on γ-ray emission for the low-energy region of the bremsstrahlung spectra produced by 800 MeV electrons in diamond crystals are presented. The obtained spectral and orientational characteristics of the γ-ray yield indicate that the low-energy radiation is associated with the channeling of electrons.     

Generation of picosecond optical phase conjugate pulses using cw mode-locked lasers

Low power picosecond optical phase conjugation has been demonstrated using cw mode-locked argon-ion and dye lasers via 4-wave mixing in ruby. Streak camera and power measurements show that the generated pulses are identical to the input pulses.     

PHOTOPHYSICAL BEHAVI0RS OF OLIGOMER BASED ON 1, 1′-BINAPHTHOL WITH 3, 3′-ACETYLENE SPACER*

The photophysical behaviors of the oligomer based on 1,1'-binaphthol with3,3'-acetylene spacer were investigated. The oligomer molecule has a naphthyl-acetylene-naphthyl effective conjugation segment. The atropic of the 1,1'-binaphthyl moiety led totwisted and rigid main chain in the oligomer. With the changes of the external environ-ment such as solvents used, solvent viscosity and ambient temperature, the wavelengthsof absorption and the intensities of fluorescence and absorption are changed slightly, butthe fluorescent intensity and quantum yield can be influenced. The luminescent behav-iors of the oligomer exhibit twisted intramolecular charge transfer characteristics, whichcould have a potential application in wavelength-stable light emitting material adaptableto ambient temperature and the solvents used in wide range.