Digital Commons Network^™

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.

Electron-Impact Ionization Of Na, A.R. Johnston, Paul Burrow

Paul Burrow Publications

Recent convergent close-coupling calculations of the ionization cross section of Na by Bray [Phys. Rev. Lett. 73, 1088 (1994)] are in poor agreement with the experimental determinations, most of which were carried out in the 1960s. We report here a measurement of the ratio of the peak ionization cross section to the cross section for excitation of the 3²P state near threshold. By normalizing the latter to the theoretical cross section computed by Moores and Norcross [J. Phys. B 5, 1482 (1972)] and more recently by Trail et al. [Phys. Rev. A 49, 3620 (1994)], we find …

State Selective Scattering Angle Dependent Capture Cross Sections Measured By Cold Target Recoil Ion Momentum Spectroscopy, Volker Mergel, Reinhard Dorner, Joachim Hermann Ullrich, Ottmar Jagutzki, Sergio Lencinas, S. Nuttgens, Lutz Spielberger, Marc Unverzagt, C. Lewis Cocke, Ronald E. Olson, Michael Schulz, Udo Buck, E. Zanger, W. Theisinger, M. Isser, S. Geis, Horst Schmidt-Bocking

Physics Faculty Research & Creative Works

We have developed a new kind of recoil ion momentum spectroscopy technique, using a precooled supersonic gas jet target, to determine state selective, scattering angle dependent cross sections for swift ion-atom collisions ( 0. 25 , ..., , 1 MeV He²⁺ on He), by measuring the transverse and longitudinal momentum of the recoil ion. A longitudinal momentum resolution of ± 0.13 a. u. was achieved, about a factor of 30 better than ever obtained before, which enables a clear separation of K and L shell capture. In the transverse direction a resolution corresponding to a projectile scattering angle uncertainty …

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 …

Electric-Field Enhancement Of Dielectronic Recombination From A Continuum Of Finite Bandwidth, J. Greg Story, Bernard J. Lyons, Thomas F. Gallagher

Physics Faculty Research & Creative Works

A small electric field is shown to increase the dielectronic recombination, via autoionizing Rydberg states, of an electron from a continuum of finite bandwidth. The continuum of finite bandwidth is a broad autoionizing state which is part of a series converging to a higher limit, and the field enhancement of the rate occurs because the field converts the nl Rydberg states to nk Stark states, increasing the number of contributing recombination paths. The experimental results are in excellent agreement with the predictions of an isolated resonance approximation treatment and show clearly both the positive effect of Stark mixing and the …

Regge Calculus As A Fourth Order Method In Numerical Relativity, Mark A. Miller

Physics - All Scholarship

The convergence properties of numerical Regge calculus as an approximation to continuum vacuum General Relativity is studied, both analytically and numerically. The Regge equations are evaluated on continuum spacetimes by assigning squared geodesic distances in the continuum manifold to the squared edge lengths in the simplicial manifold. It is found analytically that, individually, the Regge equations converge to zero as the second power of the lattice spacing, but that an average over local Regge equations converges to zero as (at the very least) the third power of the lattice spacing. Numerical studies using analytic solutions to the Einstein equations show …

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 …

Monte Carlo Studies Of A Novel Lif Radiator For Rich Detectors, Raymond Mountain, A. Efimov, Marina Artuso, Min Gao

Physics - All Scholarship

We show that a multifaceted LiF radiator produces more Cherenkov light and has better resolution per photon than a flat radiator slab when used in a ring imaging Cherenkov counter. Such a system is being considered for the CLEO III upgrade.

Three-Body Final-State Momentum Distributions For Swift H⁺And He2+On He Collisions, R. Dömer, V. Mergel, Liu Zhaoyuan, J. Uilrich, L. Spielberger, Ronald E. Olson, H. Schmidt-Bocking

Physics Faculty Research & Creative Works

The transverse and longitudinal momentum distributions of He+ions produced by 0.25-1MeV u-1H+ and He2+impact have been measured using a precooled supersonic gas jet target (cold-target recoil ion-momentum spectroscopy—coltrims). The transverse momentum of the recoil ions is mainly determined by the impact parameter. In the longitudinal direction the recoil ions are slightly backwardly directed. The width of the longitudinal momentum distribution is close to the electron momentum distribution in the initial state. This distribution gets much broader for larger transverse momentum transfer (i.e. smaller impact parameters). The experimental results are in good agreement with nCTMC calculations. © 1995 IOP Publishing Ltd.

On The Current Status Of Maser Polarization Theory, Moshe Elitzur

Physics and Astronomy Faculty Publications

Despite recent criticism, past conflicts among theoretical studies of maser polarization are by now resolved and clarified. All analytical methods proposed thus far produce the same polarization solutions, solutions that agree with observations. Current numerical studies do not reproduce these solutions. Instead, for any pumping employed, their results are in direct conflict with both analytical theory and observations. These problems reflect the fact that the basic formulation of numerical modeling has yet to incorporate the statistical nature of the radiation field.

Phase Velocity Limit Of High-Frequency Photon Density Waves, Richard C. Haskell, Lars O. Svaasand, Steen J. Madsen, Fabio E. Rojas, Ti-Chen C. Feng, Bruce J. Tromberg

All HMC Faculty Publications and Research

In frequency-domain photonmigration (FDPM), two factors make high modulation frequencies desirable. First, with frequencies as high as a few GHz, the phase lag versus frequency plot has sufficient curvature to yield both the scattering and absorption coefficients of the tissue under examination. Second, because of increased attenuation, highfrequency photon density waves probe smaller volumes, an asset in small volume in vivo or in vitro studies. This trend toward higher modulation frequencies has led us to reexamine the derivation of the standard diffusion equation (SDE)from the Boltzman transport equation. We find that a second-order time-derivative term, ordinarily neglected in the derivation, …

Method For Generating A Plasma Wave To Accelerate Electrons, Donald P. Umstadter, Eric Esarey, Joon K. Kim

Donald Umstadter Publications

The invention provides a method and apparatus for generating large amplitude nonlinear plasma waves, driven by an optimized train of independently adjustable, intense laser pulses. In the method, optimal pulse widths, interpulse spacing, and intensity profiles of each pulse are determined for each pulse in a series of pulses. A resonant region of the plasma wave phase space is found where the plasma wave is driven most efficiently by the laser pulses. The accelerator system of the invention comprises several parts: the laser system, also called beam source, which preferably comprises photo cathode electron source and RF-LINAC accelerator; electron photo-cathode …

Summary Of The Workshop On The Applications Of X-Ray Standing Waves In Studies Of Surfaces, Fiims, And Buik Materiais, David R. Heskett, L. Berman

Physics Faculty Publications

Highlights of the satellite workshop on the technique of x-ray standing waves which preceded the 5th International Conference on Synchrotron Radiation Instrumentation held at the State University of New York at Stony Brook, USA are presented. At the workshop, many applications of the x-ray standing-wave technique were described in both invited and contributed talks. While the predominant use of the XSW technique was to analyze the structure of adsorbates on surfaces, applications in the characterization of thin films, interfaces, clean surfaces, and bulk materials were also presented. Most of the talks concentrated on experimental results, but some theory was also …

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 …

Coherent Control Of H- Photodetachment In Parallel Static Electric And Magnetic Fields, Qiaoling Wang, Anthony F. Starace

Anthony F. Starace Publications

Theoretical calculations of quantum interference effects occurring in photodetachement of H- with short-pulse lasers in the presence of a uniform static electric field [Q. Wang and A. F. Starace, Phys. Rev. A 48, R1741 (1993)] are examined in more detail and extended to the case of parallel static electric and magnetic fields. We show that modulation factors characterizing near-threshold cross sections resulting from detachment by relatively long laser pulses may be employed to set the parameters that are most effective for control of detachment cross-section magnitudes with short coherent laser pulses. Use of a static magnetic field to control detached …

Grains In Ionized Nebulae: Spectral Line Diagnostics, J. B. Kingdon, Gary J. Ferland, W. A. Feibelman

Physics and Astronomy Faculty Publications

The depletion of condensable elements onto grains in gaseous nebulae can provide evidence that dust is well mixed with the ionized gas. Al and CA are two of the most depleted elements in the general interstellar medium, and it is therefore important to measure their abundances within the ionized region of nebulae. We compute a large grid of photoionization models and identify sets of line ratios which are relatively insensitive to stellar and nebular parameters, and are thus excellent diagnostics for determining relative abundances. Based on the absence of the [Ca II] λλ7291, 7324 doublet and the detection of …

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 …

Design And Performance Of The Advanced-Light-Source Double-Crystal Monochromator, G. Jones, S. Ryce, Dennis W. Lindle, B. A. Karlin, J. C. Woicik, Rupert C. Perera

Chemistry and Biochemistry Faculty Research

A new “Cowan type” double-crystal monochromator, based on the boomerang design used at National Synchrotron Light Source (NSLS) beamline X-24A, has been developed for beamline 9.3.1 at the Advanced Light Source (ALS), a windowless ultrahigh vacuum beamline covering the 1-6 keV photon-energy range. Beamline 9.3.1 is designed to simultaneously achieve the goals of high energy resolution, high flux, and high brightness at the sample. The mechanical design of the monochromator has been simplified, and recent developments in technology have been included. Measured mechanical precision of the monochromator shows significant improvement over existing designs. In tests with x-rays at NSLS beamline …

Microbes, Magnetism, And Microscopy, E. Dan Dahlberg, R. B. Proksch, Bruce M. Moskowitz, Dennis A. Bazylinski, Richard B. Frankel

Physics

An accurate quantification of magnetic force microscope images has been accomplished. The magnetosomes produced by magnetotactic bacteria, an ideal micromagnetic model system, were the specimens used for the quantification (a moment on the order of 10⁻¹³ emu).

Far-Ultraviolet Stellar Photometry: Fields In Sagittarius And Scorpius, Edward G. Schmidt, George R. Carruthers

Edward Schmidt Publications

Far-ultraviolet photometry for 741 objects in a field in Sagittarius centered near M8 and 54 1 objects in a field centered near ζ Scorpii is presented. These data were extracted from electrographic images obtained with two cameras during a shuttle flight in 1991 April/May. The cameras provided band passes with λ_eff= 1375 Å and λ_eff= 1781 Å. Synthetic colors show that these bands are sensitive to effective temperature for hot stars. Our measurements were placed on a quantitative far-ultraviolet magnitude scale by convolving the spectra of stars observed by IUE with our cameras' spectral response functions. …

Observation Of Parity-Unfavored Transitions In The Nonresonant Photoionization Of Argon, B. Langer, J. Viefhaus, Oliver Hemmers, A. Menzel, R. Wehlitz, U. Becker

Environmental Studies Faculty Publications

Single-photon ionization of an atom or molecule can be subdivided into parity-favored and -unfavored transitions, the latter characterized by electron emission, preferentially perpendicular to the electric vector. The nonresonant existence of these transitions is shown experimentally and studied over an extended energy range for a variety of satellite transitions in atomic argon. The spectra exhibit several clearly resolved satellite lines with strongly negative β values close to -1, independent of the photon energy. The results confirm the corresponding predictions of angular-momentum transfer theory.

High-Brightness Beamline For X-Ray Spectroscopy At The Advanced Light Source, Rupert C. Perera, G. Jones, Dennis W. Lindle

Chemistry and Biochemistry Faculty Research

Beamline 9.3.1 at the Advanced Light Source (ALS) is a windowless beamline, covering the 1-6 keV photon-energy range, designed to achieve the goals of high energy resolution, high flux, and high brightness at the sample. When completed later this year, it will be the first ALS monochromatic hard-x-ray beamline, and its brightness will be an order-of-magnitude higher than presently available in this energy range. In addition, it will provide flux and resolution comparable to any other beamline now in operation. To achieve these goals, two technical improvements, relative to existing x-ray beamlines, were incorporated. First, a somewhat novel optical design …

Time-Dependent Perturbation And Exact Results For The Periodically Driven Quantum Harmonic Oscillator, Russell Akridge

Faculty Articles

Transition amplitudes and probabilities for the harmonic oscillator with a forcing function proportional to cos(omegat) beginning at time zero are calculated to lowest nonvanishing order using time-dependent perturbation theory. The results are compared with the exact amplitudes and probabilities. When the exact amplitude is expanded in a Taylor series in powers of the coupling constant, the individual terms turn out to be the perturbation amplitudes, showing that the complete series of perturbation amplitudes converges to the exact amplitude.

A High Temperature Pressure Sensor Based On Magnetic Coupling And Silicon Wafer Bonding, Deguang Zhu

Theses

In this thesis, the design and fabrication of a bulk micromachined and wafer bonded pressure sensor for high temperature applications is described. The device design is based on the magnetic coupling principle as described by the Biot-Savart law. By combining the mechanical properties of single crystal silicon with magnetic coupling, the designed sensor can be operated up to 600°C. The key components within the sensor are two inductive coils, a silicon diaphragm and a hermetic vacuum cavity.

The modeling based on a nine-turn single level coil device and a 300 μm x 300 diaphragm indicates an output rms voltage range …

Ultra-Thin Silicon Wafer Bonding, Diyu Yan

Theses

In this thesis the history and recent developments on the silicon direct bonding technique are reviewed. The growing applications of this technique in SOI, SOS and MEMS areas, difficulties and disadvantages of various bonding processes are discussed. A direct bonding procedure for attaching ultra-thin wafers less than 200 μm thick to substrate wafers is developed and described in detail. Difficulties in handling, aligning and annealing ultra-thin wafers are reported. Wafers of different doping concentration, thickness, surface roughness and chemical characteristics are tested for bondability. Methods to minimize voids and other failure mechanisms are proposed. A photodetector is designed based on …