Generation of low-density plasma by two-step photoionization of barium and its evolution in electric field

Two-color, two-step resonant photoionization has been used to produce low-density (∼10⁶ – 10⁷cm⁻³) barium photoplasma in an atomic beam. The two-photon induced specific absorption coefficient at 355nm for the photoionization process has been measured. The motion of the finite size photoplasma bounded by vacuum is experimentally studied in a static electric field in parallel-plate electrode configuration. It is observed that at sufficiently high electric fields all electrons leave the plasma and the evolution of the left over ion bunch is understood on the basis of independent particle motion.     

On the determination and construction of optimal row-column designs having unequal row and column sizes

In this paper we consider experimental situations requiring usage of a row-column design where v treatments are to be applied to experimental units arranged in b ₁ rows and b ₂ columns where row i has size k _1i , i=1,..., b ₁ and column j has size k _2j , j=1,..., b ₂. Conditions analogous to those given in Kunert (1983, Ann. Statis., 11, 247–257) and Cheng (1978, Ann. Statist., 6, 1262–1272) are given which can often be used to establish the optimality of a given row-column design from the optimality of an associated block design. In addition, sufficient conditions are derived which guarantee the existence of an optimal row-column design which can be constructed by appropriately arranging treatments within blocks of an optimal block design.Visiting from the Indian Statistical Institute.     

Theory of nuclear quadrupole interactions in aluminum and copper metals in presence of muon

The electric field gradients at²⁷Al and⁶³Cu nuclei which are nearest neighbors to the muon in the face-centered cubic metals aluminum and copper, with muon at an octahedral interstitial site, are studied. The electron density fluctuations needed for the evaluation of the electronic, or valence, contributions to field gradients are taken from earlier investigations involving the spherical solid approximation. The enhancement factors a that have to be applied to the electric field-gradients obtained from these approximations, due to the departures of the electronic wave-functions from plane-wave character, and the incorporation of antishielding effects, have been obtained for both APW and OPW approaches to the conduction electron wave-functions and good agreement is found between the results by the two approaches. Size effects due to the lattice distortion associated with the presence of the muon are included through actual point ion summations using available calculated displacements of the ions surrounding the muon. The valence contributions are the dominant ones but the size effect contributions are also significant. The net field-gradient obtained for the²⁷Al site is significantly smaller than experiment while that for⁶³Cu is substantially larger than experiment. Possible sources that could lead to better agreement with experiment are discussed and it is concluded that major improvement is needed in the valence effect contributions in both metals.     

Recent Developments in Simulation Modelling

Current trends in simulation software development are towards the automation of the modelling process. Some systems provide a method of specifying the problem so that a computer program can be automatically generated. Other systems provide a generic framework for a particular class of modelling problem that can be input as data to the system. Examples of the latter are flexible manufacturing systems simulations. Most current developments are colour-graphics-aided in one form or another. Computer advances in hardware and software are emerging through better user interfaces, more power and lower costs. The major benefit is an increasing customer involvement in the modelling process. An overview of these developments and future directions are given in this paper.     

Dynamic observations of interface motion during the oxidation of silicon

We describe real time observations of the behaviour of the silicon-oxide interface during oxidation in situ in an ultrahigh vacuum transmission electron microscope. We have formed clean, flat Si(111) surfaces by heating under UHV and allowed oxidation or oxygen etching to proceed in the microscope. We have examined the kinetics of both the oxidation and etching reactions using an imaging technique based on the use of forbidden reflections in silicon. We find that oxidation to form SiO₂ occurs by the reaction of discrete monolayers with no flow of surface steps. This is in dramatic contrast to oxygen etching, during which the volatile oxide SiO evaporates preferentially from step edges.     

Chemoselective reduction of a lactam carbonyl group in the presence of a gem-dicarboxylate by sodium borohydride and iodine: a facile entry to N-aryl trisubstituted pyrroles

Several N-aryl γ-lactam gem dicarboxylates were chemoselectively reduced to cyclic amine diesters by using sodium borohydride-iodine system. The reduction in all cases was completed within 2.5 h after refluxing in THF. The cyclic amine products were isolated after aqueous (acidic) workup in good yields. Hydrolytic decarboxylation followed by dehydrogenation produced N-aryl carboethoxy pyrroles.     

Automated Processing of Two-Dimensional Correlation Spectra

An automated scheme is described which locates the centers of cross peaks in two-dimensional correlation spectra, even under conditions of severe overlap. Double-quantum-filtered correlation (DQ-COSY) spectra have been investigated, but the method is also applicable to TOCSY and NOESY spectra. The search criterion is the intrinsic symmetry (or antisymmetry) of cross-peak multiplets. An initial global search provides the preliminary information to build up a two-dimensional “chemical shift grid.” All genuine cross peaks must be centered at intersections of this grid, a fact that reduces the extent of the subsequent search program enormously. The program recognizes cross peaks by examining the symmetry of signals in a test zone centered at a grid intersection. This “symmetry filter” employs a “lowest value algorithm” to discriminate against overlapping responses from adjacent multiplets. A progressive multiplet subtraction scheme provides further suppression of overlap effects. The processed two-dimensional correlation spectrum represents cross peaks as points at the chemical shift coordinates, with some indication of their relative intensities. Alternatively, the information is presented in the form of a correlation table. The authenticity of a given cross peak is judged by a set of “confidence criteria” expressed as numerical parameters. Experimental results are presented for the 400-MHz double-quantum-filtered COSY spectrum of 4-androsten-3,17-dione, a case where there is severe overlap.