Showing papers by "National Institute of Standards and Technology published in 1992"

Chemical kinetics and photochemical data for use in stratospheric modeling

Abstract: As part of a series of evaluated sets, rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation are provided. The primary application of the data is in the modeling of stratospheric processes, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena. Copies of this evaluation are available from the Jet Propulsion Laboratory.

Spin squeezing and reduced quantum noise in spectroscopy.

Abstract: We investigate the quantum-mechanical noise in spectroscopic experiments on ensembles of N two-level (or spin-1/2) systems where transitions are detected by measuring changes in state population. By preparing correlated states, here called squeezed spin states, we can increase the signal-to-noise ratio in spectroscopy (by approximately ${\mathit{N}}^{1/2}$ in certain cases) over that found in experiments using uncorrelated states. Possible experimental demonstrations of this enhancement are discussed.

Taguchi's parameter design: a panel discussion

Abstract: It is more than a decade since Genichi Taguchi's ideas on quality improvement were inrroduced in the United States. His parameter-design approach for reducing variation in products and processes has generated a great deal of interest among both quality practitioners and statisticians. The statistical techniques used by Taguchi to implement parameter design have been the subject of much debate, however, and there has been considerable research aimed at integrating the parameter-design principles with well-established statistical techniques. On the other hand, Taguchi and his colleagues feel that these research efforts by statisticians are misguided and reflect a lack of understanding of the engineering principles underlying Taguchi's methodology. This panel discussion provides a forum for a technical discussion of these diverse views. A group of practitioners and researchers discuss the role of parameter design and Taguchi's methodology for implementing it. The topics covered include the importance of vari...

Oxidative damage to DNA in mammalian chromatin.

Abstract: Efforts have been made to characterize and measure DNA modifications produced in mammalian chromatin in vitro and in vivo by a variety of free radical-producing systems. Methodologies incorporating the technique of gas chromatography/mass spectrometry have been used for this purpose. A number of products from all four DNA bases and several DNA-protein cross-links in isolated chromatin have been identified and quantitated. Product formation has been shown to depend on the free radical-producing system and the presence or absence of oxygen. A similar pattern of DNA modifications has also been observed in chromatin of cultured mammalian cells treated with ionizing radiation or H2O2 and in chromatin of organs of animals treated with carcinogenic metal salts.

Spin Squeezing and Reduced Quantum Noise in Spectroscopy

Abstract: We investigate the quantum-mechanical noise in spectroscopic experiments on ensembles of N two-level (or spin-1/2) systems where transitions are detected by measuring changes in state population. By preparing correlated states, here called squeezed spin states, we can increase the signal-to-noise ratio in spectroscopy (by approximately ${\mathit{N}}^{1/2}$ in certain cases) over that found in experiments using uncorrelated states. Possible experimental demonstrations of this enhancement are discussed.

Relevance feedback revisited

Abstract: Researchers have found relevance feedback to be effective in interactive information retrieval, although few formal user experiments have been made. In order to run a user experiment on a large document collection, experiments were performed at NIST to complete some of the missing links found in using the probabilistic retrieval model. These experiments, using the Cranfield 1400 collection, showed the importance of query expansion in addition to query reweighting, and showed that adding as few as 20 well-selected terms could result in performance improvements of over 100%. Additionally it was shown that performing multiple iterations of feedback is highly effective.

Magnetic excitations in pure, lightly doped, and weakly metallic La2CuO4.

Abstract: We report a comprehensive neutron-scattering study of the evolution of the magnetic excitations in La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4] for 0[le][ital x][le]0.04. We first present accurate measurements of the magnetic correlation length and the sublattice magnetization of a carrier-free La[sub 2]CuO[sub 4] crystal and analyze these in the context of recent theoretical predictions. We then systematically investigate the influence of different dopants on the magnetism: Our measurements indicate that static vacancies in the La[sub 2]Cu[sub 1[minus][ital y]]Zn[sub [ital y]]O[sub 4] system affect the magnetic correlations in a similar manner as electrons in Pr[sub 2[minus][ital x]]Ce[sub [ital x]]CuO[sub 4]. The magnetic correlation length is much more rapidly suppressed as a function of [ital x] in La[sub 2[minus][ital x]]Sr[sub [ital x]]CuO[sub 4], and for [ital x][le]0.04 we find that it obeys the empirical relation [xi][sup [minus]1]([ital x],[ital T])=[xi][sup [minus]1]([ital x],0)+[xi][sup [minus]1](0,[ital T]), where [xi](0,[ital T]) is the measured correlation length of the carrier-free sample. We also report an extensive set of measurements of the dynamical magnetic response function of a crystal of composition La[sub 1.96]Sr[sub 0.04]CuO[sub 4] for excitation energies 0.75[le][omega][le]45 meV and temperatures 1.5[le][ital T][le]500 K.

The NIST x‐ray photoelectron spectroscopy database

Abstract: The National Institute of Standards and Technology (NIST) has made available the second version of the NIST X-ray photoelectron Spectroscopy Database. This paper reports on recent upgrades to the database to make the information more up-to-date and to make data searching and display more convenient and efficient for the user. Additional data are included in the database together with the capability to search using large chemical synonym lists, chemical formulae and other chemical classification schemes

A General Waveguide Circuit Theory.

Abstract: This work generalizes and extends the classical circuit theory of electromagnetic waveguides. Unlike the conventional theory, the present formulation applies to all waveguides composed of linear, isotropic material, even those involving lossy conductors and hybrid mode fields, in a fully rigorous way. Special attention is given to distinguishing the traveling waves, constructed with respect to a well-defined characteristic impedance, from a set of pseudo-waves, defined with respect to an arbitrary reference impedance. Matrices characterizing a linear circuit are defined, and relationships among them, some newly discovered, are derived. New ramifications of reciprocity are developed. Measurement of various network parameters is given extensive treatment.

Computer simulation of the diffusivity of cement-based materials

Abstract: A digital image-based model of the microstructure of cement paste, coupled with exact transport algorithms, is used to study the diffusivity of Portland cement paste. The principal variables considered are water∶cement ratio, degree of cement hydration and capillary porosity. Computational methods are described and diffusivity results are presented, which are found to agree with the available experimental measurements within experimental error. Model cement pastes prepared with different water∶cement ratios, and having different degrees of hydration, are found to have diffusivities that lie on a single master curve when plotted as a function of capillary porosity. Concepts from percolation theory are used to explain quantitatively the dependence of diffusivity on capillary porosity. The effect of silica fume addition on diffusivity is also examined.

Calculation of the background emitted during high-harmonic generation

Abstract: We present a method for calculating the spectrum emitted by an atom in an intense laser field. This method is based on the direct use of the acceleration of the atomic electron rather than the dipole moment of the atom. We show, using a numerical calculation in one dimension, that this method produces a more exact numerical evaluation of the spectrum. This technique is particularly important in determining the background to the high harmonics emitted by the atom.

Contact Electrification and Adhesion Between Dissimilar Materials

Abstract: Simultaneous measurements of surface force and surface charge demonstrate strong attraction due to the spontaneous transfer of electrical charge from one smooth insulator (mica) to another (silica) as a result of simple, nonsliding contact in dry nitrogen. The measured surface charge densities are 5 to 20 millicoulombs per square meter after contact. The work required to separate the charged surfaces is typically 6 to 9 joules per square meter, comparable to the fracture energies of ionic-covalent materials. Observation of partial gas discharges when the surfaces are approximately 1 micrometer apart gives valuable insight into the charge separation processes underlying static electrical phenomena in general.

Ionic crystals in a linear Paul trap

Abstract: We describe a configuration for a linear Paul rf ion trap. This trap can store a long string of ions with a small second-order Doppler shift, comparable to that achieved with a single ion in a quadrupole Paul trap. Crystallized strings of trapped ions, as well as more complicated structures, have been observed in the trap. We report an observation of the 40.5-GHz ground-state hyperfine interval of $^{199}\mathrm{Hg}^{+}$ by microwave-optical double-resonance spectroscopy and discuss prospects for a microwave frequency standard based on a trapped string of ions.

Electron production in proton collisions with atoms and molecules: energy distributions

Abstract: All known data on the energy distribution of secondary electrons from collisions of protons with atoms and molecules have been reviewed and differential cross sections have been collected. The two experimental methods used to obtain the data are discussed and possible sources of error pointed out. Theoretical treatments are reviewed and several methods of checking the consistency of the data are discussed. Two semiempirical models have been chosen to represent the differential cross sections, and parameters for these models are given which fit the average of the experimental data, subject to known constraints. Recommended values of differential cross sections are given for ten target gases by means of these models.

Overview No. 98 I—Geometric models of crystal growth

Abstract: Recent theoretical advances in the mathematical treatment of geometric interface motion make more tractable the theory of a wide variety of materials science problems where the interface velocity is not controlled by long-range-diffusion. Among the interface motion problems that can be modelled as geometric are certain types of phase changes, crystal growth, domain growth, grain growth, ion beam and chemical etching, and coherency stress driven interface migration. We provide an introduction to nine mathematical methods for solving such problems, give the limits of applicability of the methods, and discuss the relations among them theoretically and their uses in computation. Comparisons of some of them are made by displaying how the same physical problems are treated in the various applicable methods.

The measurement of oxidative damage to DNA by HPLC and GC/MS techniques.

Abstract: Oxidative damage to DNA has been measured by quantitating 8-hydroxy-2′-deoxyguanosine (8-OHdGuo) after enzymic digestion of DNA, followed by HPLC separation and electrochemical detection. Alternatively, 8-hydroxyguanine (and a wide range of other base-derived products of free radical attack) may be measured after acidic hydrolysis of DNA or chromatin, followed by derivatization and gas-chromatography/mass spectrometry. Both techniques have comparable sensitivity, but GC/MS enables determination of a wide variety of chemical changes to all four DNA bases and it can be applied to DNA-protein complexes. However, the two techniques do not always give similar results. Potential reasons for this are discussed. Greater attention to methodological questions is required before using measurement of 8-OHdGuo as a “routine” marker of oxidative DNA damage in vivo.

Laser cooling of trapped ions in a standing wave.

Abstract: Laser cooling of trapped ions in a standing- and running-wave configuration is discussed theoretically. The ions are assumed to be spatially localized on the scale provided by the wavelength of the laser (Lamb-Dicke limit). A master equation for the center-of-mass distribution of the ion is derived for a multilevel system, and explicit results are presented for two- and three-level systems and harmonic trapping potentials. For the two-level system located at the node of the standing wave, we find final temperatures that are a factor of 2 lower than the limit for a running wave and cooling rates that do not saturate with the laser intensity. At the point of maximum gradient of the standing wave, blue detuned cooling is found that is analogous to the Sisyphus cooling of free atoms. For a three-level system we compare our results with those of Wineland, Dalibard, and Cohen-Tannoudji [J. Opt. Soc. Am. B 9, 32 (1992)].

Observation of quantized motion of Rb atoms in an optical field.

Abstract: We observe transitions of laser-cooled Rb between vibrational levels in subwavelength-sized optical potential wells, using high-resolution spectroscopy of resonance fluorescence. We measure the spacing of the levels and the population distribution, and find the atoms to be localized to 1/15 of the optical wavelength. We find up to 60% of the population of trapped atoms in the vibrational ground state. The dependence of the spectrum on the parameters of the optical field provides detailed information about the dynamics of laser-cooled atoms.

An experimental investigation of the pulsation frequency of flames

Abstract: Measurements of the pulsation frequency in non-premixed flames were conducted for gaseous and liquid fuels. Measurements were performed over a wide range of Froude number (≈10−4 to ≈103), Reynolds number (≈10 to ≈103) and burner diameter (0.0074 m to 0.30 m). The fuel velocity at the burner exit had a weak influence on the pulsation frequency for some diameters. The Strouhal number plotted as a function of the inverse Froude number was shown to correlate the measurements determined here as well as measurements reported in the literature for pulsations in flames burning gaseous, liquid and solid fuels over 14 orders of magnitude in Froude number. Previous measurements of the effect of enhanced gravitational level on the pulsation frequency were also interpreted in terms of the Strouhal number—Froude number relationship. Stability limits in flames were investigated. The critical fuel velocity needed to initiate pulsations was measured as a function of burner diameter for methane and propane flames. Flow visualization was used to measure the pulsation frequency of an isothermal buoyant gas stream over a range of Froude number (10−3 to 1). A plot of the Strouhal number as a function of the inverse Froude number also correlated the measurements, but yielded a different power law exponent than the reacting flow case.

The elastic moduli of a sheet containing circular holes

Abstract: W e apply computer simulation techniques to obtain the clastic moduli of a matrix containing circular holes. As the area fraction of holes increases, the Young's modulus of the composite decreases from E0 until it eventually vanishes at the percolation threshold. We study three distinct geometries: (a) periodically centered circular holes on a honeycomb lattice, (b) periodically centered circular holes on a triangular lattice, and (c) randomly centered circular holes. All three cases have the same dilute limit that can be calculated exactly. By examining the narrow necks between adjacent circles, we have calculated the critical behavior for the regular cases and obtain critical exponents of 1 2 or 3 2 , depending on the local breakdown mode at the necks. For (c) we compare our results with an effective-medium theory, which predicts that the Poisson's ratio tends to 1 3 as the percolation threshold is approached, independent of its value in the pure system. Our results are also compared with recent experimental results. Based on this work, we propose that the relative Young's modulus E E 0 of a two-dimensional sheet containing circular holes, overlapping or not, is the same for ail materials, independent of the Poisson's ratio v0, for any prescribed geometry.

Diffraction anomalous fine structure: A new x-ray structural technique.

Abstract: A new x-ray structural technique, diffraction anomalous fine structure (DAFS), which combines the long-range order sensitivity of diffraction techniques with the short-range order sensitivity of absorption techniques, is described. We demonstrate that synchrotron DAFS measurements for the Cu(111) and Cu(222) Bragg reflections provide the same local atomic structural information as x-ray absorption fine structure and describe how DAFS can be used to provide enhanced site and spatial sensitivities for polyatomic and/or spatially modulated structures

Ordering of thin diblock copolymer films.

Abstract: The ordering in thin films of symmetric diblock copolymers has been studied by neutron reflectivity as a function of temperature and film thickness. Exponentially damped oscillatory variations in the concentration are shown to propagate into the specimens from both surfaces with decay lengths increasing with decreasing temperature. No distinguishable order-to-disorder transition is observed; only a transition from a partially to fully ordered state is found. The temperature of this transition is found to depend in a power-law manner on the film thickness.