Digital Commons Network^™

Conduction-Electron Screening In The Bulk And At Low-Index Surfaces Of Ta Metal, D. Mark Riffe, W. Hale, B. Kim, J. L. Erskine

All Physics Faculty Publications

High-resolution core-level photoemission spectra from Ta(100) have been measured. The well-resolved peak from the first atomic layer allows a separate assessment of bulk and surface-layer screening responses: singularity indices α_B=0.10±0.01 and α₁₀₀=0.205±0.025, respectively, are obtained. The measured surface-atom lifetime broadening of 70±20 meV (compared to 37±5 meV in the bulk) is consistent with published Auger–photoemission coincidence measurements. The result for the bulk singularity index has been applied in the analysis of previous data in order to extract the screening response in the first atomic layer of Ta(111) (α₁₁₁=0.16±0.01) and Ta(110) (α₁₁₀=0.150±0.015). The …

Scaling And The Fractal Geometry Of Two-Dimensional Quantum Gravity, Simon Catterall, G. Thorleifsson, Mark Bowick, V. John

Physics - All Scholarship

We examine the scaling of geodesic correlation functions in two-dimensional gravity and in spin systems coupled to gravity. The numerical data support the scaling hypothesis and indicate that the quantum geometry develops a non-perturbative length scale. The existence of this length scale allows us to extract a Hausdorff dimension. In the case of pure gravity we find d_H approx. 3.8, in support of recent theoretical calculations that d_H = 4. We also discuss the back-reaction of matter on the geometry.

On Science Schooling, Seminar Style, E. H. Weinberg, I. Semeniuk, Peter J. Collings, G. Greenstein

Physics & Astronomy Faculty Works

No abstract provided.

Collisional Effects In He I: An Observational Analysis, J. B. Kingdon, Gary J. Ferland

Physics and Astronomy Faculty Publications

Accurate and reliable helium abundances can test modern theories of galactic and primordial nucleosynthesis. Unfortunately, there is some question whether current theory can account for collisional contributions to He I. We present new observations of two planetary nebulae (PNs) in the range λ850-λ9650, which we use to assess the importance of collisonal effects in the He I spectrum. The first object, NGC 7027, is expected to show relatively strong collisional enhancement, while the second, NGC 7026, should display only small effects. We derive new collision-to-recombination correction factors, based on new collision strengths from the 29-state quantal calculation …

An Experimental Apparatus For Studying Differential Scattering Of Multiply-Charged Ions From Neutral Targets Following Capture, John Edward Edens

Masters Theses

An experimental apparatus has been designed, enabling measurement of angular differential cross sections for single-electron capture (SEC). The design consists of a microchannel plate detector with a one-dimensional position sensitive anode. A bow-time shaped aperture was employed to convert a radially scattered distribution into an approximate linear one. An electrostatic retarding grid was used to separate SEC contributions from the reaction products. Measurements were made of the angular differential cross sections for SEC of Ar^q+ (q=4-6,8) ions scattering from He and Ar at impact energies of 600 to 3000 eV and angles between 0 and 23 mrad. The experimental …

Wing-Section Optimization For Supersonic Viscous Flow, Cem Cihan Item

Mechanical & Aerospace Engineering Theses & Dissertations

The recent interest in the High Speed Civil Transport (HSCT) has resulted in renewed research studies of optimized supersonic cruise transport configurations. Incorporation of flow viscosity effects in the design process of such a supersonic wing is currently under investigation. This may lead to more accurate problem formulations and, in tum, greater aerodynamic efficiency than can be obtained by the traditional, inviscid, linear theories. In this context, for a design code to be a candidate for a complex optimization problem, such as three-dimensional viscous supersonic wing design, it should be validated using simpler building-block shapes.

To optimize the shape of …

Measurement Of Hyperfine Coupling Constants Of The 5d²D₃/₂ And 5d²D₅/₂ Levels In Atomic Cesium Using Polarization Quantum Beat Spectroscopy, Wo Yei

Physics Theses & Dissertations

Accurate measurements of hyperfine constants have revealed effects that can not be explained by a simple hydrogenic picture of the alkali atoms such as cesium [1-3]. More precise experimental results and theoretical treatments are in demand for the alkali elements, especially for atomic cesium because of its wide range of applications. Therefore, it is essential to understand its atomic and nuclear structure. Precision measurement of its excited-states properties such as hyperfine structure provides global information on nuclear charge and current distributions and also serves as a check to the theory and a calibration of calculated excited state wave functions. Accurate …

Thermal Effects On Y2O3:Eu3+ For Fiber Optic Temperature Sensing, Arnel C. Lavarias

Electrical & Computer Engineering Theses & Dissertations

Fiber optic methods have gained acceptance in the past several years as non­ intrusive optical techniques of measuring environmental variables, particularly under hostile conditions. Fiber optic temperature sensors are required for many applications which have a large operating range. One way to implement this sensor is to introduce rare earth-doped materials into a fiber showing temperature-dependent optical emission characteristics. The thermal effects on optical emission characteristics of Y₂O₃:Eu³⁺ were studied. The thermal broadening and temperature dependence of the lifetime (τ) of the 611.4 nm emission due to the ⁵D₀ ➔ 7F …

Excited-State Magnesium-Rare Gas Optical Collisions, Rosemary A. Lasell

Physics Theses & Dissertations

Polarization and intensity spectra have been obtained for excited-state magnesium (3s3p ¹P₁) - rare gas optical collisions. The magnesium was excited by a linearly polarized laser tuned to the 3s²1s₀ → 3s3p ¹P₁ atomic transition at 285 nm. A second linearly polarized laser was tuned near the 3s3p ¹P₁ → 3s5s ¹S₀ transition at 571 nm (experiment A) and near the 3s3p ¹P₁ → 3s4d ¹D₂ transition at 553 nm (experiment B). The polarization vector of the second laser was oriented either parallel or …

Measure Of Fine Tuning, Greg W. Anderson, Diego Castano

Chemistry and Physics Faculty Articles

Fine-tuning criteria are frequently used to place upper limits on the masses of superpartners in supersymmetric extensions of the standard model. However, commonly used prescriptions for quantifying naturalness have some important shortcomings. Motivated by this, we propose new criteria for quantifying fine tuning that can be used to place upper limits on superpartner masses with greater fidelity. In addition, our analysis attempts to make explicit the assumptions implicit in quantifications of naturalness. We apply our criteria to the minimal supersymmetric extension of the standard model, and we find that the scale of supersymmetry breaking can be larger than previous methods …

Heavy Quark Solitons: Towards Realistic Masses, Joseph Schechter, A. Subbaraman, S. Vaidya, H. Weigel

Physics - All Scholarship

A generalization of the effective meson Lagrangian possessing the heavy quark symmetry to finite meson masses is employed to study the meson mass dependence of the spectrum of S-- and P wave baryons containing one heavy quark or anti-quark. These baryons are described as respectively heavy mesons or anti-mesons bound in the background of a soliton, which is constructed from light mesons. No further approximation is made to solve the bound state equation. For special cases it is shown that the boundary conditions, which have to be satisfied by the bound state wave--functions and stem from the interaction with the …

Resolving Ordering Ambiguities In The Collective Quantization By Particle Conjugation Constraints, Joseph Schechter, H. Weigel

Physics - All Scholarship

We formulate the particle conjugation operation and its convenient realization as G--parity in the framework of several chiral soliton models. The Skyrme model, the Skyrme model with vector mesons and the chiral quark model are specifically treated. The vector and axial vector currents are classified according to their behavior under G--parity. In the soliton sector particle conjugation constrains {\it a priori} ambiguous orderings of operators in the space of the collective coordinates. In the Skyrme model with vector mesons and in a local chiral model with an explicit valence quark this classification scheme provides consistency conditions for the ordering of …

Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. I. 20- 114-Kev H+ + H2, M. W. Gealy, G. W. Kerby, Iii, Y.-Y. Hsu, M. Eugene Rudd

M. Eugene Rudd Publications

Apparatus and procedures are described for the measurement of absolute cross sections, differential in ejected electron energy and angle, for ionization of atomic and molecular hydrogen by ion impact. A hemispherical electrostatic energy analyzer, rotatable from 15° to 165° with respect to the direction of the incident ion beam, was used to measure energy spectra of secondary electrons from 1.5 to 300 eV. Cross sections at ten angles (nine at some energies) and five incident-ion energies from 20 to 114 keV for H⁺ +H₂ collisions are given. The doubly differential cross sections were integrated over angle and electron …

Challenging Weak-Scale Supersymmetry At Colliders, Greg W. Anderson, Diego Castano

Chemistry and Physics Faculty Articles

Experimental searches for supersymmetry are entering a new era. As future experiments explore the mass range above the current lower bounds on superpartner masses, a failure to observe signals of superpartner production will begin to erode the central motivation for supersymmetry at the weak scale. In this article we present a detailed examination of which regions of supersymmetric parameter space are most natural and the extent to which weak-scale supersymmetry becomes unnatural if no superpartners are observed at CERN LEP II, the Fermilab Tevatron, possible upgrades of these machines, and the CERN LHC.

Mass Distribution On Clusters At The Percolation Threshold, M. F. Gyure, M. V. Ferer, Boyd F. Edwards, G. Huber

All Physics Faculty Publications

Monte Carlo simulations and a scaling hypothesis are used to study the distribution of blob masses on two-dimensional finite-mass clusters at the percolation threshold. The exponents associated with this distribution function are a combination of backbone and percolation exponents. This work offers insights into the structure and fragmentation properties of percolation clusters in particular, and provides methods applicable to other fractal distribution problems in general.

Percus-Yevick Approximation For Fluids With Spontaneous Partial Order, Hua Zhong, Rolfe G. Petschek

Faculty Scholarship

In this paper we present a Percus-Yevick approximation which can be applied to a system in which a phase transition has occurred, resulting in spontaneous breaking of a continuous symmetry and partial spontaneous order. Previous methods have allowed calculations only for molecules which are in the isotropic phase or all perfectly aligned. For the orientationally disordered isotropic phase, our technique gives identical results to previous work. The appropriate treatment for systems with spontaneously broken continuous symmetry has been appreciated for some time in the magnetic and field theoretic literature. We adapt these treatments to the anisotropic fluid system with a …

Differential Double-Excitation Cross Sections In 50-150-Kev Proton-Helium Collisions, Michael Schulz, W. T. Htwe, A. D. Gaus, Jerry Peacher, Tibor Vajnai

Physics Faculty Research & Creative Works

We have measured projectile-energy-loss spectra for 50-, 100-, and 150-keV p+He collisions. From the data we obtained differential double-excitation cross sections as a function of projectile scattering angle. At 150 keV a pronounced peak structure was observed at about 0.7 mrad for double excitation to the (2p²) ¹D and (2s2p) ¹P states. Our data provide indications for the dominance of a first-order mechanism involving the electron-electron interaction in double excitation for 150 keV at small scattering angles. At lower projectile energies and larger scattering angles a second-order mechanism appears to be of the same order of …

Energy And Angular Distributions Of Electrons From Ion Impact On Atomic And Molecular Hydrogen. Ii. 20- 114-Kev H+ + H, G. W. Kerby, Iii, M. W. Gealy, Y.-Y. Hsu, M. Eugene Rudd, D. R. Schultz, C. O. Reinhold

M. Eugene Rudd Publications

Results of crossed-beam measurements of cross sections differential in ejected electron energy and angle for ionization of atomic hydrogen by 20-1 14-keV protons are reported. Secondary electrons were measured over an energy range of 1.5-300 eV and an angular range of 15°-165°. Atomic-hydrogen targets were produced in a radio-frequency discharge source with a dissociation fraction of about 74%. Ratios of cross sections for H targets to those for Hz targets were obtained from measurements on the mixed target. From these ratios, the measured dissociation fractions, and the absolute cross sections measured for H, targets, the cross sections for H targets …

Dissociative Attachment In Hot Ch3Cl: Experiment And Theory, D. M. Pearl, Paul Burrow, Ilya I. Fabrikant, Gordon A. Gallup

Paul Burrow Publications

The dissociative attachment (DA) cross section of hot CH₃Cl has been measured in a crossed electron–molecule beam apparatus at temperatures up to 750 K and electron energies from 0–0.5 eV. The results are compared to cross sections computed using a mixed ab initio-semiempirical approach, treating CH₃Cl as a quasidiatomic molecule. The theoretical treatment requires an anion potential curve in the stable region as a portion of the input data. Computations with three different basis sets show the results to be sensitive to the size of basis set from which the potential is determined. At high …

Performance Characterization Of The Caltech Compact Torus Injector, P. K. Loewenhardt, Michael R. Brown, J. Yee, P. M. Bellan

Physics & Astronomy Faculty Works

A device is described which is designed to refuel tokamaks of major radius [approximately-equal-to]1 m and magnetic field [approximately-equal-to]1 T by the injection of a compact torus. The injector utilizes an acceleration stage which adds kinetic energy to the compact torus so that it can penetrate the tokamak magnetic field. The performance of the device was found to be optimized in an unexpected parameter regime. It is theorized that in this regime the accelerator takes part in the formation of the CT. A performance threshold with respect to accelerator current will be presented and methods of impurity reduction will be …

Heavy Ion Track-Structure Calculations For Radial Dose In Arbitrary Materials, Francis A. Cucinotta, Robert Katz, John W. Wilson, Rajendra R. Dubey

Robert Katz Publications

The δ-ray theory of track structure is compared with experimental data for the radial dose from heavy ion irradiation. The effects of electron transmission and the angular dependence of secondary electron ejection are included in the calculations. Several empirical formulas for electron range and energy are compared in a wide variety of materials in order to extend the application of the track-structure theory. The model of Rudd for the secondary electron spectrum in proton collisions, which is based on a modified classical kinematics binary encounter model at high energies and a molecular promotion model at low energies, is employed. For …

Convective Chemical-Wave Propagation In The Belousov- Zhabotinsky Reaction, Y. Wu, D. A. Vasquez, Boyd F. Edwards, J. W. Wilder

All Physics Faculty Publications

We investigate the onset of convection for chemical-wave propagation in the Belousov-Zhabotinsky reaction based on the two-variable Oregonator model coupled with the fluid dynamic equations. For chemical waves in a vertical slab, two-dimensional convection occurs only for slab widths greater than a critical threshold width. The convective threshold is different for ascending and descending waves. Convectionless waves are flat and propagate with constant speed. Above the onset of convection, the wave velocity increases and the flat wave deforms due to two counterrotating steady rolls. For a horizontal slab, convection is always present and the wave velocity increases with increasing slab …

Convenient Determination Of Concentration And Energy In Deep-Level Transient Spectroscopy, David C. Look, Z-Q. Fang, J. R. Sizelove

Physics Faculty Publications

For defects or impurities with deep energy levels, such as the commonly observed EL2, EL3, and EL6 in GaAs, it is very important to take account of the so‐called λ effect in order to deduce the correct concentrations of these centers when using capacitance techniques. By measuring capacitance at several forward bias voltages for a given reverse bias voltage it is possible to determine concentration N_T and energy E_T without requiring the usual emission rate analysis. Convenient formulas for N_T and E_T are given, although only N_T can be determined with a high degree of …

Exploring \Pp Scattering In The \1n Picture, Joseph Schechter, Francesco Sannino

Physics - All Scholarship

In the large N_c approximation to QCD, the leading \pp scattering amplitude is expressed as the sum of an infinite number of tree diagrams. We investigate the possibility that an adequate approximation at energies up to somewhat more than one GeV can be made by keeping diagrams which involve the exchange of resonances in this energy range in addition to the simplest chiral contact terms. In this approach crossing symmetry is automatic but individual terms tend to drastically violate partial wave unitarity. We first note that the introduction of the \rho meson in a chirally invariant manner substantially delays the …

Improved Gaussian Beam-Scattering Algorithm, James A. Lock

Physics Faculty Publications

The localized model of the beam-shape coefficients for Gaussian beam-scattering theory by a spherical particle provides a great simplification in the numerical implementation of the theory. We derive an alternative form for the localized coefficients that is more convenient for computer computations and that provides physical insight into the details of the scattering process. We construct a FORTRAN program for Gaussian beam scattering with the localized model and compare its computer run time on a personal computer with that of a traditional Mie scattering program and with three other published methods for computing Gaussian beam scattering. We show that the …

Linear Transport Theory Of Magnetoconductance In Metallic Multilayers: A Real-Space Approach, Horacio E. Camblong

Physics and Astronomy

The linear transport behavior of metallic multilayers is studied by using the real-space Kubo formula, for a model Hamiltonian consisting of zero-range spin-dependent impurity-scattering potentials. The resulting theory allows a direct comparison with the Boltzmann equation approach in the quasiclassical limit, which is expected to be a good approximation for most magnetic multilayers. Furthermore, the regimes for which quantum corrections might be needed are explicitly indicated. It is explicitly shown that: (i) periodicity is not required as a mechanism for giant magnetoresistance, and (ii) the two main geometries, current in the plane of the layers and perpendicular to the plane …

State-Selected Ion-Molecule Reactions: Charge Transfer And Atomic Rearrangement Processes In Thermal Energy Collisions Of H2+ (X;V)+N2 And Of N2+ (X,A;V) + H2, Cornelis J. Uiterwaal, J. Van Eck, A. Niehaus

C.J.G.J. Uiterwaal Publications

Using the photo-electron-product-ion-coincidence method (PEPICO) we have measured state-selective cross sections for the following processes: (A) N₂⁺ (X,A;v)+ H₂→N₂H⁺ + H, (B) H₂⁺ (X;v) + N₂→N₂H⁺ + H, (C) N₂⁺ (X,A;v)+H₂→H₂⁺ + N₂, and (D) H₂⁺ (X;v)+N₂→N₂⁺ + H₂ . The measurements were performed at thermal velocities (E_c.m. ≈ 40 meV).We have found that the charge transfer processes (C) and (D) …

Comment On 'Simulation Of A Two-Dimensional Rayleigh-Bénard System Using The Direct Simulation Monte Carlo Method, Alejandro Garcia, F. Baras, M. Malek Mansour

Faculty Publications

No abstract provided.

Radio-Frequency Superconducting Parametric Transducer For Gravitational-Wave Antennae, M. A. Fisher, Mark F. Bocko, Linda E. Marchese, Guizhen Zhang, Munawar Karim

Physics Faculty/Staff Publications

We report on the design and testing of an ultrasensitive, electromechanical transducer for use on resonant mass gravitational wave antennae. The transducer is a superconducting, radio frequency resonant bridge circuit operating near 200 MHz. We have minimized several important sources of noise in this transducer system. The Johnson noise of the transducer circuit is reduced through using a superconducting niobium stripline circuit and low‐loss insulating materials. At a temperature of 4.2 K we have achieved unloaded electrical quality factors of 200 000. The bridge circuit is balanced by piezoelectric actuators which control the spacing between the proof mass and capacitive …

Mcplib: A Collection Of Nonlinear Mixed Complementarity Problems, Steven P. Dirkse, Michael C. Ferris

University Faculty Publications and Creative Works

The origins and some motivational details of a collection of nonlinear mixed complementarity problems are given. This collection serves two purposes. Firstly, it gives a uniform basis for testing currently available and new algorithms for mixed complementarity problems. Function and Jacobian evaluations for the resulting problems are provided via a GAMS interface, making thorough testing of algorithms on practical complementarity problems possible. Secondly, it gives examples of how to formulate many popular problem formats as mixed complementarity problems and how to describe the resulting problems in GAMS format. We demonstrate the ease and power of formulating practical models in the …