Crystal deformation during the growth of kink bands in oriented polyethylene

When oriented polyethylene is sheared at an angle to the orientation axis, kink bands often develop and grow, with a resulting change of the crystalline orientation. Beside the crystalline reorientation, the following changes within the kink bands have been observed with wide-angle x-rays: (a) partial transformation to a monoclinic from the normal orthorhombic unit cell; (b) partial alignment of the orthorhombic b axes; (c) rotation of the orthorhombic c axes of a fraction of the crystals around the kinks by an extra 40 to 60° beyond that of the fibrils; and (d) misalignment of the orthorhombic (hk0) planes by a few degrees. These results are suggested to arise, at least in part, from crystal flattening and from crystal twinning or pseudotwinning on planes intersecting the molecular axes.     

Formation of fluoro-meisenheimer complexes using homogeneous and heterogeneous fluoride ion sources

1-Chloro-2,4-dinitrobenzene, 1-fluoro-2,4-dinitrobenzene and 1,3-dinitrobenzene form Meisenheimer complexes with tetra-n-butylammonium fluoride and alumina supported alkali metal fluorides. Both kinetically controlled and thermodynamically controlled complexes are observed.     

Fluorescence detection of exafs: Sensitivity enhancement for dilute species and thin films

The fluorescence intensity is used to measure the X-ray absorption cross-section and is found to yield essentially the same results as a more conventional transmission experiment. However, the fluorescence method is shown to extend the sensitivity of the EXAFS technique by two or more orders of magnitude, and thus nake feasible the study of extremely dilute species.     

The structure of amorphous GeSe and GeTe

We present the results of electronic structure calculations on model 3:3 and 4:2 co-ordinated networks of amorphous GeSe and GeTe. The calculations show that a- GeTe has a predominantly 4:2 chemically ordered structure and suggest that GeSe may be 3:3 co-ordinated. They emphasize how it is often easier to discriminate between different models on the basis of electronic structure rather than other information.     

Semi-chaotic pulse effects in self-modulating Gunn oscillators

Self-modulation behaviour has been observed in GaAs Gunn oscillators operating at W-band, leading to nanosecond pulses of relatively high peak power. Pulse repetition rates and pulse-widths can be varied reproducibly as a function of bias voltage and bias line termination, and there is evidence for semi-chaotic behaviour in certain regimes.     

Spatial redistribution of energy in a nanosecond laser pulse by an organic optical limiter

The spatial redistribution of energy resulting from the interaction between a near-diffraction-limited nanosecond laser pulse and the nonlinear absorbing optical limiting dye silicon naphthalocyanine is described, for what is to our knowledge the first time, in an optical geometry that is likely to be found in practical applications. For input fluences above that required for nonlinear absorption but below that for bubble growth, a plane wave or Gaussian spatial input evolves unexpectedly to a sharp central spike and a well-defined outer ring. The observed energy redistribution is thought to rely on a combination of nonlinear processes, since a pure absorptive process alone cannot explain the profiles presented. A model involving nonlinear absorption and nonlinear refraction qualitatively reproduces the observed spatial profiles. It is clear from the results that the performance of optical limiting dyes in representative optical geometries, even at fluences well below that required for bubble growth, cannot be described solely by nonlinear absorption.     

2.5-GHz repetition-rate singly resonant optical parametric oscillator synchronously pumped by a mode-locked diode oscillator amplifier system

We report on an optical parametric oscillator (OPO) that is synchronously pumped directly by a diode laser. This laser is an actively mode-locked master-oscillator power-amplifier system that produces 20-ps pulses at 927 nm with a repetition rate of 2.5 GHz and an average power of 0.9 W. The OPO, which is a singly resonant device based on periodically poled lithium niobate, generates 7.8-ps pulses. The OPO threshold is 300 mW of average pump power, and the maximum average idler output power is 78 mW at a wavelength of 2100 nm. By changing the crystal temperature we can wavelength tune the output in the ranges 1530-1737 nm (signal) and 1986-2348 nm (idler). Rapid wavelength tuning of the OPO over 46 nm (signal) and 74 nm (idler) is achieved through tuning the cavity length over 28 microm by use of a piezoelectric transducer.     

MOON for neutrino-less double beta decays

The MOON (Majorana/Mo Observatory Of Neutrinos) project aims at studies of the Majorana nature of the neutrino (ν) and the ν-mass spectrum by spectroscopic experiments of neutrino-less double beta decays (0νββ) with the ν-mass sensitivity of 〈m _ν ^m 〉 = 100−30 meV. The solid scintillator option of the MOON detector is a super ensemble of multi-layer modules, each being composed by PL scintillator plates and position-sensitive detector planes with good overall energy resolution of σ ≈ 2% at the Q _ββ ≈ 3 MeV. Thin ββ source films are interleaved between the detector planes. High localization of the two β tracks enables one to select true signals and reject BG ones. The multi-layer structure of the detector makes it realistic to build a compact ton-scale detector. MOON with detector ≠ ββ source is used for studying 0νββ decays from ¹⁰⁰Mo, ⁸²Se and other ββ isotopes with large Q _ββ . Real-time exclusive measurements of low energy solar neutrinos can be made by observing inverse β rays from solar-ν captures of ¹⁰⁰Mo in delayed coincidence with the subsequent β decay of ¹⁰⁰Tc.     

Large harmonic softening of the phonon density of states of uranium

Phonon density-of-states curves were obtained from inelastic neutron scattering spectra from the three crystalline phases of uranium at temperatures from 50 to 1213 K. The alpha-phase showed an unusually large thermal softening of phonon frequencies. Analysis of the vibrational power spectrum showed that this phonon softening originates with the softening of a harmonic solid, as opposed to vibrations in anharmonic potentials. It follows that thermal excitations of electronic states are more significant thermodynamically than are the classical volume effects. For the alpha-beta and beta-gamma phase transitions, vibrational and electronic entropies were comparable.     

A new method of stirring for LPE growth

This paper describes a method of stirring a liquid during crystal growth in which each point on the growth face moves round a circle of radius b. If the face has a radius a and completes n revolutions per second, then the stirring is roughly equivalent to rotating the disc about a single axis at a rate $\text{2πc}{}^2\text{b}{}^2\text{n}\text{a}{}^2$ where c is a constant (c ∽ 2) but the system behaves as if only the liquid up to a distance $\text{(}\text{2v}\text{n}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ from the face is stirred. Uniform stirring occurs over the whole face except for an annulus of width 2b at the periphery. The necessary design criteria are discussed and a simple dynamically balanced apparatus and the results obtained with it whilst growing iron garnet films are described.