External Operator Research Articles

Plasmons in simple metal slabs: a semi-classical approach

Collective excitations in simple metal systems can be successfully described in terms of a local one-body excitation operator Q, due to the long-range nature of the Coulomb interaction. For the plasmon modes of a simple metal slab, momentum expansions of Q are calculated using a variational procedure equivalent to a restricted RPA calculation. The dispersion relation and the density fluctuation for each mode are found in the sudden approximation using the proper Q operator and the RPA sum rule formalism. The contributions of the exchange and correlation energy are estimated using a local density functional. The positive background is described within a jellium model, while the ground-state electronic density is approximated by a double step profile. The density fluctuation of the plasmon modes above the plasma frequency form standing waves across the slab. The spectra below the plasma frequency are qualitatively different to those of local optics calculations due to the appearance of two multipole plasmon modes which shift the origin of the ω + plasmon down. The dependence of the results on the width of the slab, the density of the simple metal and the surface diffuseness is discussed. The response of the slab to a q- dependent external excitation operator is analyzed. Throughout, the difference with to RPA and TDLDA calculations is stressed.

Functional characterization of roles of GalR and GalS as regulators of the gal regulon.

An isorepressor of the gal regulon in Escherichia coli, GalS, has been purified to homogeneity. In vitro DNase I protection experiments indicated that among operators of the gal regulon, GalS binds most strongly to the external operator of the mgl operon, which encodes the high-affinity beta-methylgalactoside galactose transport system, and with less affinity to the operators controlling expression of the gal operon, which codes for enzymes of galactose metabolism. GalS has even less affinity for the external operator of galP, which codes for galactose permease, the major low-affinity galactose transporter in the cell. This order of affinities is the reverse of that of GalR, which binds most strongly to the operator of galP and most weakly to that of mgl. Our results also show that GalS, like its homolog, GalR, is a dimeric protein which in binding to the bipartite operators of the gal operon selectively represses its P1 promoter. Consistent with the fact that GalR is the exclusive regulator of the low-affinity galactose transporter, galactose permease, and that the major role of GalS is in regulating expression of the high-affinity galactose transporter encoded by the mgl operon, we found that the DNA binding of GalS is 15-fold more sensitive than that of GalR to galactose.

An Ada-based script language for simulation applications

This paper describes SLIS, an Ada-based script language used in the simulations of Fokker's Test Centre. First an overview of the simulation development process, its goals and the resulting requirements for the command interface of simulation modules are presented. Then, the separation of a simulation's internal implementation from the external operator view is discussed. The architecture of SLIS and its interaction with simulation modules is described next. Finally, the script language itself is presented.

Increasing Oil Production Through Leasing of Idle Wells

The national oil company of Trinidad and Tobago (Petrotrin) has more than 5,000 idle wells. The company initiated a procedure for leasing these idle wells to small independent operators in July 1989 through a program known as the lease operatorship program. This paper reviews the status of lease operatorship activities, examines the benefits and the strategies adopted by the operators as well as some of the important issues involved, describes the requirements for external operatorship, and outlines planes for the future of the program.

Static polarizability associated with multipole surface plasmons in metallic surfaces.

An Euler-Lagrange-type equation is solved to different orders for a plane metallic system perturbed by the external operator Q^ used previously in a sum-rule calculation. This approach managed to reproduce to within a few percent the multipole surface-plasmon energies. The jellium model is used for the positive ionic background and local-density functionals for the Hamiltonian. From the zeroth-order Euler-Lagrange equation a self-consistent ground-state density is obtained and used in the first-order equation to obtain the induced density. From the induced density, several aspects of the system response are considered and special attention is focused on the static polarizability.

Interaction of a regulatory protein with a DNA target containing two overlapping binding sites

The LexA repressor from Escherichia coli regulates the transcription of about 20 different genes upon binding to single or multiple operators. In this work we study the interaction of LexA with the control region of the caa gene (coding for the bacterial toxin colicin A) that contains two operators (O1 and O2) which overlap by at least 2 base pairs relevant for sequence specific DNA recognition. This arrangement raises the question of how the LexA molecules which bind to the central overlapping part of the two operators avoid steric clashes and further, of whether the interaction of LexA with the two operators is cooperative or not. To address these questions we have constructed two mutant operators (O1+O2- and O1-O2+) for which the two most strongly conserved base pairs in each of the external operator half-sites have been mutated. Using methylation interference with the complex formation of LexA with the wild-type and these two mutant operators we could show: 1) that the two mutant operators behave symmetrically in that the methylation of one crucial guanine base in both operator half-sites interferes strongly with complex formation, 2) but that in the wild-type operator (containing four functional operator half-sites) only the two external half-operators give rise to interference if this crucial guanine base is methylated, whereas methylation of the two equivalent guanine bases within the two central (overlapping) operator half-sites does not lead to interference with the formation of a complex where both operators are occupied simultaneously. These data suggest that the centrally bound LexA molecules adopt a somewhat different binding mode than those bound to the external half-operators in order to avoid steric clashes and/or to optimize protein-protein contacts which are likely to be at the origin of the binding cooperativity that we could demonstrate by quantitative DNase I footprinting and gel retardation experiments. While the methylation interference experiments revealed a non-equivalence for the binding of externally and centrally bound LexA molecules, both methylation protection and hydroxyl radical footprinting were unable to reveal this difference, suggesting that the difference between the two binding modes should be fairly subtle.

Velocity Measurements In Convective Flows By Particle Image Velocimetry Using A Low Power Laser

To date, powerful lasers such as argon or ruby lasers have been used in most applications of particle image velocimetry. We present an analysis of the ability of this technique when a low power (5 mW He-Ne) laser is used. As a practical example we apply it to the Rayleigh-Benard convective flow, where velocities ranging up to several mm/s have been measured with about 100µJ energy pulses. The parameters that allow us to obtain good quality photographs with such low light power are analyzed. Several examples of pictures taken from different flow planes at different Rayleigh numbers are shown. The data reduction is made by fringe analysis. The digital image of the Young's fringes is recorded with a videocamera at each point of the photograph and is then processed with the 1-D averaging technique. This technique is the fastest but has the drawback of needing an external operator to select the fringe direction. To avoid this problem, we implement an algorithm whose practical performance is analyzed here. Another improvement proposed for this technique is the pedestal removal from the signal by digital high pass filtering. In this way, automatic detection of the first peak in the Fourier transform of the 1-D averaged signal is reached more easily. Thus, full automation of the 1-D averaging technique is obtained. The spatial filtering technique is also used. Several examples of whole-field velocity maps obtained from the photographs are shown, allowing a quick analysis of the velocity field. Finally, results obtained from postprocessing of the velocity measurements are shown. They include velocity, vorticity, and streamline maps. Isovelocity contour maps are also generated with the computer from the velocity field to compare with the maps obtained from the spatial filtering technique.

A search for integrable two degree of freedom Hamiltonians systems with polynomial potential : Jarmo Hietarinta, Department of Physical Sciences, University of Turku, 20500 Turku 50, FINLAND

Collective excitations in simple metal systems can be successfully described in terms of a local one-body excitation operator Q, due to the long-range nature of the Coulomb interaction. For the plasmon modes of a simple metal slab, momentum expansions of Q are calculated using a variational procedure equivalent to a restricted RPA calculation. The dispersion relation and the density fluctuation for each mode are found in the sudden approximation using the proper Q operator and the RPA sum rule formalism. The contributions of the exchange and correlation energy are estimated using a local density functional. The positive background is described within a jellium model, while the ground-state electronic density is approximated by a double step profile. The density fluctuation of the plasmon modes above the plasma frequency form standing waves across the slab. The spectra below the plasma frequency are qualitatively different to those of local optics calculations due to the appearance of two multipole plasmon modes which shift the origin of the ω+ plasmon down. The dependence of the results on the width of the slab, the density of the simple metal and the surface diffuseness is discussed. The response of the slab to a q-dependent external excitation operator is analyzed. Throughout, the difference with to RPA and TDLDA calculations is stressed.

A multivariable selftuning regulator to control a double effect evaporator

A multivariable selftuning regulator has been applied in control of a pilot plant double effect evaporator. The selftuning regulator consists of a recursive least squares algorithm combined with a single step optimal control strategy which minimizes a quadratic criterion. The algorithm is very fast and can be adjusted by an external operator in order to obtain satisfactory stationary regulator performance.

Nuclear field theory of Fermi systems in an external field

Nuclear field theory (NFT) is developed for interacting Fermi systems in an external field by using path integral techniques. The NFT representation of the external field operator together with the corresponding diagrammatic rules are strictly derived.