有机硅活性中间体的研究 II. 一些含环丙基的有机硅化合物的合成及其结构

用锂有机物的方法合成了四种含有环丙基有机硅化合物,对上述每种化合物中可能存在着的立体异构进行了分离和和构型测定。     

NEAR ULTRAVIOLET INACTIVATION STUDIES ONESCHERICHIA COLI TRYPTOPHANASE AND TRYPTOPHAN SYNTHETASE

Abstract— –The response of two pyridoxal-phosphate-requiring enzymes of E. coli, tryptophanase and tryptophan synthetase, to near UV light (320–400 nm) has been studied. Tryptophanase is inactivated both in vivo and in vitro, but tryptophan synthetase is resistant to near UV under both conditions. This shows that near UV inactivation is not general for pyridoxal-phosphate-requiring enzymes. Substrate protection against light inactivation is demonstrated for tryptophanase. It is furthermore shown that pyridoxal phosphate is required for inactivation of this enzyme. However, the action spectrum for this inactivation does not coincide with the absorption spectrum of tryptophanase or of pyridoxal phosphate.     

Spectroscopic evidence for bonding coordination defects in amorphous as

Spectroscopic evidence for four-fold and possibly, two-fold coordinated defects in amorphous As is presented. Structure in Raman scattering and infrared absorption measurements is interpreted using force-constant models and the cluster-Bethe-lattice method. The positions and intensities of certain peaks suggest that these defects may be the analogues of valence alternation pairs in amorphous chalcogenide glasses.     

A review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry—I. Fundamental principles of signal-to-noise ratios

This review consists of two parts which discuss signal-to-noise in a tutorial manner. The sources of noise, the mathematical representation of noise, and the major types of noises in emission and luminescence spectrometry are discussed. An extensive treatment of noise and signal-to-noise ratios of paired readings is given using the relation between the auto-correlation function and the spectral noise power. These signal-to-noise expressions under optimized measurement conditions are given in terms of currents and count rates as well as in terms of charge and counts for the cases of d.c. and a.c. measurements; the present treatment is limited to the cases when the background shows only either shot noise or flicker noise. Finally, the consequences of the combination of these noise sources is considered. Signal expressions for optical spectrometry are also given. Tables give the expressions for signal-to-noise ratios in the various cases.     

Turbulent pipe flow manipulation: some experimental and computational results for single manipulator rings

The effect on developing turbulent pipe flow of blade manipulator devices has been investigated both experimentally and computationally with a view towards reducing pipeline transmission losses. Wall pressure and mean axial velocity profiles have been obtained for nine manipulator configurations. Numerical calculations have been performed for two of these manipulator configurations using a finite volume method that incorporates a low Reynolds number model of turbulence. The results suggest that nett drag reduction may be possible in this flow although at present this seems unlikely. It remains to be answered whether any benefit can be achieved in fully developed flow for which both experimental and computational studies are now underway.     

Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16D × 21.05D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios (J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow.

The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow.

In the opposed jet configuration at the high momentum flux ratio (J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.     

Distinguishing nanocrystalline solids from fine-particle ensembles using Mössbauer spectroscopy

Nanocrystalline solids, formed by compacting fine powders, have been proclaimed to possess a new structure. Specific characteristics have been reported from Mössbauer studies, and these appear to confirm and reinforce a model of crystalline cores isolated by disordered, low-density grain boundaries. In measurements of three compacted samples, we were unable to reproduce the characteristics expected. Reasons are suggested as to why the results differ between similarly-prepared samples. It is pertinent to enquire whether any of the previously reported Mössbauer characteristics of fine-particle compacts require a new structural model, or whether all the results can be simply understood in terms of particle agglomerates. Reference to the literature reveals that there is no distinction.     

Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

Quantum key distribution using quantum-correlated photon sources

Quantum key exchanges using weak coherent (Poissonian) single-photon sources are open to attack by a variety of eavesdropping techniques. Quantum-correlated photon sources provide a means of flagging potentially insecure multiple-photon emissions and thus extending the secure quantum key channel capacity and the secure key distribution range. We present indicative photon-counting statistics for a fully correlated Poissonian multibeam photon source in which the transmitted beam is conditioned by photon number measurements on the remaining beams with non-ideal multiphoton counters. We show that significant rejection of insecure photon pulses from a twin-beam source cannot be obtained with a detector having a realistic quantum efficiency. However quantum-correlated (quadruplet or octuplet) multiplet photon sources conditioned by high efficiency multiphoton counters could provide large improvements in the secure channel capacity and the secure distribution range of high loss systems such as those using the low earth orbit satellite links proposed for global quantum key distribution. Received 14 July and Received in final form 20 November 2001