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Articles 31081 - 31110 of 34249

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Solid State Proton Spin Relaxation And T-Butyl And Methyl Group Reorientation In 1-Bromo-2,4,6-Tri-T-Butylbenzene, Anne M. Fry, Peter A. Beckmann, Albert J. Fry, Peter C. Fox, Ari Isenstadt Oct 1991

Solid State Proton Spin Relaxation And T-Butyl And Methyl Group Reorientation In 1-Bromo-2,4,6-Tri-T-Butylbenzene, Anne M. Fry, Peter A. Beckmann, Albert J. Fry, Peter C. Fox, Ari Isenstadt

Physics Faculty Research and Scholarship

We have used the solid state proton spin relaxation technique to investigate the barriers for methyl and t-butyl group reorientation in polycrystalline 1-bromo-2,4,6-tri-t-butylbenzene. The barriers in the range of 15-19 kJ/mol (3-5 kcal/mol) are compared with those found in related molecules. It is shown that the neighboring ring bromine atom has an effect on the barrier for t-butyl group reorientation similar to that of a neighboring hydrogen atom despite the significantly larger van der Waals' radius of a bromine atom. This most likely occurs because of the relatively long carbon-bromine bond, the distorted ring geometry, and the relatively high polarizability …

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Scaling Behavior Of Atomic Trajectories In Confined Fluids, Amy Lisa Graves, B. J. Berne Oct 1991

Scaling Behavior Of Atomic Trajectories In Confined Fluids, Amy Lisa Graves, B. J. Berne

Physics & Astronomy Faculty Works

We study the dependence of D, a quantity that has previously been associated with the fractal dimension of an atomic trajectory, on the scale on which it is measured. Single-particle and relative trajectories are generated by molecular-dynamics simulations of Lennard-Jones atoms permanently trapped in a spherical pore. Transient trapping is studied with a generalized Langevin model of dynamics. Confinement of trajectories allows D to exceed 2; in the case of permanent trapping, D diverges.

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Internal Caustic Structure Of Illuminated Liquid Droplets, James A. Lock, Edward A. Hovenac Oct 1991

Internal Caustic Structure Of Illuminated Liquid Droplets, James A. Lock, Edward A. Hovenac

Physics Faculty Publications

The internal electric field of an illuminated liquid droplet is studied in detail with the use of both wave theory and ray theory. The internal field attains its maximum values on the caustics within the droplet. Ray theory is used to determine the equations of these caustics and the density of rays on them. The Debye-series expansion of the interior-field Mie amplitudes is used to calculate the wave-theory version of these caustics. The physical interpretation of the sources of stimulated Raman scattering and fluorescence emission within a liquid droplet is then given.

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Inorganic Particles Produced By Microorganisms, Richard B. Frankel Oct 1991

Inorganic Particles Produced By Microorganisms, Richard B. Frankel

Physics

The processes by which microorganisms, particularly bacteria, produce inorganic mineral particles are reviewed.

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Structure, Morphology And Growth Of Biogenic Greigite (Fe3S4), Brigid R. Heywood, Stephen Mann, Richard B. Frankel Oct 1991

Structure, Morphology And Growth Of Biogenic Greigite (Fe3S4), Brigid R. Heywood, Stephen Mann, Richard B. Frankel

Physics

Several species of aquatic bacteria are known to exploit the earth's geomagnetic field as a means of directing their motion towards suitable habitats. A feature common to these bacteria is the presence of discrete intracellular magnetic inclusions, magnetosomes, aligned in chains along the long axis of the organism. The size and orientation of the individual magnetic particles imparts a permanent magnetic dipole moment to the cell which is, in turn, responsible for the magnetotactic response. In all species examined to date the magnetic particles have been found to be well-ordered, single domain, membrane-bounded crystals with reproducible, species-specific morphologies. Until recently, …

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Study Of Cathodoluminescence In Semiconductors, Mandakini V. Kale Oct 1991

Study Of Cathodoluminescence In Semiconductors, Mandakini V. Kale

Electrical & Computer Engineering Theses & Dissertations

Cathodoluminescence in Zn doped GaAs and ZnSe, the materials of interest for electron-beam controlled semiconductor switches, has been investigated. A new diagnostic technique, using telescopic setup of lenses, was developed to study the spatially resolved cathodoluminescence. The setup eliminated the light, other than rays from the sample, parallel to the optical axis, resulting in an enhanced spatial resolution. At high e-beam energies, light was detected, from the Zn layer region in the GaAs. It is proposed to be due to the radiative recombination of electron-hole pairs, created by the e-beam. For the polycrystalline ZnSe no light emission was detected. This …

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Auxiliary Field In Conformal Gauge Theory, James Thomas Wheeler Sep 1991

Auxiliary Field In Conformal Gauge Theory, James Thomas Wheeler

All Physics Faculty Publications

When the full conformal algebra is gauged there arises a gauge field for inverse translations in addition to the usual translational gauge field. By considering every scale-invariant action constructible from the curvatures of the conformal group and the metric, we show that when the gauge field of the usual translations is identified as the vierbein, the gauge field of inverse translations may always be eliminated by its own field equation. After this elimination, every torsion-free, scale-invariant action reduces to a linear combination of the square of the conformal curvature tensor Cαβμν and the square of the Weyl field strength …

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Track Physics Model Of Radiation Effects, Robert Katz Sep 1991

Track Physics Model Of Radiation Effects, Robert Katz

Robert Katz Publications

Radiation effects induced by heavy ions in many materials with diverse end points are well described by the conceptual structure and equations of track physics, first developed for heavy ion tracks in nuclear emulsions. The model describes scintillators, biological cell inactivation and mutation, radiation chemistry, latent tracks in insulators, the response of resists to heavy ions, and other systems. A detector is taken to be composed of small targets whose response to ionizing radiation is principally to secondary electrons. The response is calibrated through determination of the probability of target (in)activation as a function of the absorbed dose of γ …

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Gaseous Component Identification With Polymeric Film Sensor, Edward S. Kolesar Jr. Sep 1991

Gaseous Component Identification With Polymeric Film Sensor, Edward S. Kolesar Jr.

AFIT Patents

A sensor having an interdigitated gate electrode field effect transistor (IGEFET) coupled to an electron beam evaporated copper phthalocyanine thin film is used to selectively detect parts-per-billion concentration levels of atmosphere contaminants such as nitrogen dioxide (NO2) and diisopropyl methylphosphonate (DIMP). The sensor is excited with a voltage pulse, and its time- and frequency-domain response are examined. The envelopes of the magnitude of the normalized difference frequency spectrums reveal features which unambiguously distinguish the NO2 and DIMP exposures.

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Can An Economic Approach Solve The High-Level Nuclear Waste Problem, Herbert Inhaber Sep 1991

Can An Economic Approach Solve The High-Level Nuclear Waste Problem, Herbert Inhaber

RISK: Health, Safety & Environment (1990-2002)

Building on the work of Professors O'Hare and Kunreuther, Dr. Inhaber proposes and argues for a non-coercive siting strategy that he calls a "reverse Dutch auction."

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Π‾P→Π⁰Π⁰N Near Threshold And Chiral Symmetry Breaking, William J. Fickinger, D. K. Robinson Sep 1991

Π‾P→Π⁰Π⁰N Near Threshold And Chiral Symmetry Breaking, William J. Fickinger, D. K. Robinson

Faculty Scholarship

Total cross sections, angular, and mass distributions for the reaction π-p→π0π0n have been measured for pπ-(lab)=7–140 MeV/c above threshold. The threshold amplitude was used to determine a value for the chiral-symmetry-breaking parameter, ξ, of -0.98±0.52. The ππ scattering lengths aI for isospin I=0 and 2 are derived from this result, together with a current-algebra sum rule. The results are a0=(0.207±0.028)mπ-1 and a2=(-0.022±0.011)mπ-1. These values are consistent with chiral symmetry broken by the Weinberg ππ interaction and the effects of the f0(975) scalar meson.

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On Compensation And Conductivity Models For Molecular-Beam-Epitaxial Gaas Grown At Low-Temperature, David C. Look Sep 1991

On Compensation And Conductivity Models For Molecular-Beam-Epitaxial Gaas Grown At Low-Temperature, David C. Look

Physics Faculty Publications

Molecular‐beam‐epitaxial GaAs grown at 200 °C has an extremely high (≳1019 cm−3) concentration of AsGa defects and, after an anneal at 550–600 °C, a high concentration of As precipitates. The relative roles of the AsGa defects and As precipitates in compensation and conductivity is controversial. Here criteria are developed to distinguish between two existing models.

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Aggregation Of Aqueous Suspensions Of Phospholipid Tubules, Min-Hua Lu, Jerome B. Lando, J. Adin Mann Jr., Rolfe G. Petschek, Charles Rosenblatt Sep 1991

Aggregation Of Aqueous Suspensions Of Phospholipid Tubules, Min-Hua Lu, Jerome B. Lando, J. Adin Mann Jr., Rolfe G. Petschek, Charles Rosenblatt

Faculty Scholarship

[No abstract available]

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Paper Or Plastic?, Matthew J. Moelter Sep 1991

Paper Or Plastic?, Matthew J. Moelter

Physics

Letter to the Editor.

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Mie Theory Model Of The Corona, James A. Lock, Leiming Yang Aug 1991

Mie Theory Model Of The Corona, James A. Lock, Leiming Yang

Physics Faculty Publications

We performed a calculation of the corona colors that employed Mie theory to obtain the scattered light intensity. The scattered intensity was integrated over the visible spectrum for a number of different cloud droplet size distributions. The results were converted to chromaticity coordinates, convolved with the angular size of the sun, and plotted on the 1931 CIE chromaticity diagram. The results were compared to observations of multiple-ring coronas. It was found that, when using Mie theory to estimate cloud droplet sizes, water droplets with diameters in the 7-mu-m less-than-or-similar-to D less-than-or-similar-to 15-mu-m range produced the 13 multiple-ring coronas that were …

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Light And Color In The Open Air: Introduction By The Feature Editor, James A. Lock, Craig F. Bohren Aug 1991

Light And Color In The Open Air: Introduction By The Feature Editor, James A. Lock, Craig F. Bohren

Physics Faculty Publications

No abstract provided.

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An Experimental Investigation Of The Finite Time Efficiency Of A Peltier Refrigeration Device, Thomas Schneider Aug 1991

An Experimental Investigation Of The Finite Time Efficiency Of A Peltier Refrigeration Device, Thomas Schneider

Dissertations and Theses

Since the need of energy conservation has become more and more urgent in the past decades, there has been an increased interest in the study and development of more efficient energy conversion systems. One of the fields that have arisen from that endeavor is a branch of physics called Finite Time Thermodynamics (FIT). It may be said that FIT was initiated through the famous paper by Curzon and Ahlborn (1975) that established new bounds on the efficiency of a finite time Carnot heat engine. Before, the traditional treatments gave a fundamental upper limit on the efficiency of any heat engine. …

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Binary Peak Enhancement And Structure In Partially Stripped Ion-Atom Collisions, D. R. Schultz, Ronald E. Olson Aug 1991

Binary Peak Enhancement And Structure In Partially Stripped Ion-Atom Collisions, D. R. Schultz, Ronald E. Olson

Physics Faculty Research & Creative Works

Recent experiments have found both an unexpected enhancement and structure of the binary peak in the spectrum of electrons ejected in partially stripped ion-atom collisions. Utilizing the theoretical approach which has been used to elucidate the origin of this behavior, a survey of their manifestation is presented over a broad range of nuclear and ionic charges (C, F, Fe, I and U ions) and impact energies (0.1 to 100 MeV u-1). For forward binary electron emission it is shown that the greatest enhancement occurs for the lowest projectile ion charge states (highly screened nuclei). The magnitude of the enhancement maximizes …

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Differential And Total Cross Sections For Ionization Of Helium And Hydrogen By Electrons, M. Eugene Rudd Aug 1991

Differential And Total Cross Sections For Ionization Of Helium And Hydrogen By Electrons, M. Eugene Rudd

M. Eugene Rudd Publications

A comprehensive semiempirical model for singly and doubly differential and total ionization cross sections for electron production by electron impact is presented. The model is discussed in relation to several theoretically required constraints. Available experimental data for hydrogen and helium targets are examined, and recommended values of the data are given for all ranges of primary and secondary energies and angles of ejection.

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Laboratory Simulations Of Suprauroral Mechanisms Leading To Perpendicular Ion Heating And Conic Formation, D. P. Sheehan, R. Koslover, R. Mcwilliams Aug 1991

Laboratory Simulations Of Suprauroral Mechanisms Leading To Perpendicular Ion Heating And Conic Formation, D. P. Sheehan, R. Koslover, R. Mcwilliams

Physics and Biophysics: Faculty Scholarship

Laboratory experiments are presented simulating aspects of perpendicular ion heating and conic formation that are observed or hypothesized to occur in the terrestrial ionosphere and magnetosphere. Previous laboratory observations of ion conics in the presence of the current‐driven electrostatic ion cyclotron wave are reviewed. Field‐aligned ion beams, accompanied by beam‐generated electrostatic ion cyclotron modes, resulted in perpendicular energization of beam ions and also the heating of background plasma ions. Antenna‐launched broadband and narrow‐band lower hybrid waves produced considerable perpendicular ion heating and non‐Maxwellian “tail” formation. Laboratory results are discussed in light of in situ measurements by the S3‐3 satellite and …

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Ultrafast Electronic Disordering During Femtosecond Laser Melting Of Gaas, Peter N. Saeta, J.-K. Wang, Y. Siegal, N. Bloembergen, E. Mazur Aug 1991

Ultrafast Electronic Disordering During Femtosecond Laser Melting Of Gaas, Peter N. Saeta, J.-K. Wang, Y. Siegal, N. Bloembergen, E. Mazur

All HMC Faculty Publications and Research

We have observed an ultrarapid electronic phase transformation to a centrosymmetric electronic state during laser excitation of GaAs with intense femtosecond pulses. Reflection second-harmonic intensity from the upper 90 atomic layers vanishes within 100 fs; reflectivity rises within 0.5 ps to a steady value characteristic of a metallic molten phase, long before phonon emission can heat the lattice to the melting temperature.

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Frequency Shifts Of Molecules At Rough Metal Surfaces, M. H. Hider, P.T. Leung Aug 1991

Frequency Shifts Of Molecules At Rough Metal Surfaces, M. H. Hider, P.T. Leung

Physics Faculty Publications and Presentations

The frequency shifts for dipolar transitions of molecules at rough metallic surfaces are studied in a phenomenological model following the approach of the authors' previous work [Phys. Rev. B 36, 4664 (1987)], with the surface roughness modeled by a shallow grating profile in most cases. The main findings here are as follows: (1) such surface-induced shifts are indeed observable for molecular frequencies away from the surface-plasmon resonance frequency of the metal; (2) the presence of surface roughness can either enhance or suppress the flat-surface-induced shifts, leading to extra morphology-dependent resonances originated from the radiative coupling between the molecular emission and …

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Measurement Of The Ratio Of Double-To-Single Photoionization Of Helium At 2.8 Kev Using Synchrotron Radiation, Jon C. Levin, Dennis W. Lindle, N. Keller, R. D. Miller, Y. Azuma, N. Berrah Mansour, H. G. Berry, Ivan A. Sellin Aug 1991

Measurement Of The Ratio Of Double-To-Single Photoionization Of Helium At 2.8 Kev Using Synchrotron Radiation, Jon C. Levin, Dennis W. Lindle, N. Keller, R. D. Miller, Y. Azuma, N. Berrah Mansour, H. G. Berry, Ivan A. Sellin

Chemistry and Biochemistry Faculty Research

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He++/He+=1.6±0.3% at hν=2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Åberg, who predict 1.7%.

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Anisotropy Of Polarized X-Ray Emission From Molecules, S. H. Southworth, Dennis W. Lindle, R. Mayer, P. L. Cowan Aug 1991

Anisotropy Of Polarized X-Ray Emission From Molecules, S. H. Southworth, Dennis W. Lindle, R. Mayer, P. L. Cowan

Chemistry and Biochemistry Faculty Research

Strongly anisotropic, polarized Cl K-V x-ray emission from gas-phase CF3Cl has been observed following resonant excitation with a linearly polarized x-ray beam. Distinctively different angular distributions are observed for x-ray emission involving molecular orbitals of different symmetries. A classical model of the x-ray absorption-emission process accurately describes the observed radiation patterns.

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Collisional Dynamics Of The B3Π(O+) State Of Bromine Of Monofluoride, David W. Melton Aug 1991

Collisional Dynamics Of The B3Π(O+) State Of Bromine Of Monofluoride, David W. Melton

Theses and Dissertations

When Bromine is combined with flourine atoms and discharged oxygen in a flow tube is produced in a highly non thermal distribution peaking at v'=3. The emission was linearly dependent on the concentration. The observed dependence of the BrF(B) emission on Bromine and atomic oxygen is inconsistent with a steady state prediction of a sequential excitation mechanism, where BrF(X) is excited by successive collisions with singlet oxygen. The experimental data are consistent with a three body mechanism involving Br and F atoms. Electronic quenching of BrF(B) by a number of collision partners (Br2, CO2, O2 …

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X-Ray Emission From Slow Highly Charged Ar Ions Interacting With A Ge Surface, Michael Schulz, C. Lewis Cocke, Siegbert Hagmann, Martin P. Stockli, Horst Schmidt-Bocking Aug 1991

X-Ray Emission From Slow Highly Charged Ar Ions Interacting With A Ge Surface, Michael Schulz, C. Lewis Cocke, Siegbert Hagmann, Martin P. Stockli, Horst Schmidt-Bocking

Physics Faculty Research & Creative Works

We have measured K x-ray spectra and yields from Ar17+ ions slowly approaching a single-crystal Ge surface. The yields were measured as a function of the projectile velocity component perpendicular to the surface. From the data a characteristic time of approximately 1 psec was extracted for the in-flight filling above the surface of the Ar K vacancy.

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A Track Physics Model Of Radiation Action, Robert Katz Aug 1991

A Track Physics Model Of Radiation Action, Robert Katz

Robert Katz Publications

A model of radiation action by energetic heavy ions, inspired by a study of particle tracks in electron sensitive emulsion, has been successfully applied to many radiation effects in condensed matter, yielding quantitative descriptions and occasional predictions. Radiation effects are attributed primarily to secondary electrons. Each detector is imagined to be a collection of targets whose radiosensitivity is represented parametrically. We avoid a mechanistic description of detector response so as to retain the global character of the model. Attention is focused on the structure of particle tracks, on the radial deposition of dose about an ion’s path by “δ-rays,” and …

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Inhibited Spontaneous Emission By Photonic Band Gaps In A Square Lattice Of Periodic Dielectric Medium, Abdullah Al-Ghamdi Aug 1991

Inhibited Spontaneous Emission By Photonic Band Gaps In A Square Lattice Of Periodic Dielectric Medium, Abdullah Al-Ghamdi

Masters Theses

The concepts of band theory for electrons can be employed to describe the behavior of electromagnetic waves propagating in two-dimensional, periodic dielectric structures. These two-dimensional, periodic structures can produce photonic band gaps in which the spontaneous emission by atoms embedded in the structure is prohibited and as a result the performance of many semiconductor devices can be enhanced. The calculations are based on finding the eigenvalues of the algebraic equation for the frequencies of electromagnetic waves moving in a square lattice composed of square dielectric rods with different dielectric constants.

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Eta Photoproduction Via 2H(Γη)2H*, Yan Zhang Aug 1991

Eta Photoproduction Via 2H(Γη)2H*, Yan Zhang

Masters Theses

The reaction H(γη)2H* is studied using the impulse approximation. A recent fit to the H(γη)2H* elementary amplitude is employed in the calculation. The n-p final state is described by Reid's soft core potentials. The calculation shows very different cross sections for the final isospin 0 and isospin 1 break-up channels. The S11 resonance is demonstrated to dominate the eta cross sections in the 740 MeV photon laboratory energy region. Fermi motion in the deuteron does not significantly spread the effect of the S11 . All these results demonstrate that the reaction H(γη) …

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Electronic And Magnetic Structures Of The Rare-Earth Compounds R2Fe17NΞ, Sitaram Jaswal, W.B. Yelon, George C. Hadjipanayis, Y.Z. Wang, David J. Sellmyer Jul 1991

Electronic And Magnetic Structures Of The Rare-Earth Compounds R2Fe17NΞ, Sitaram Jaswal, W.B. Yelon, George C. Hadjipanayis, Y.Z. Wang, David J. Sellmyer

David Sellmyer Publications

Structural and magnetic properties of the rare-earth compounds R2Fe17Nξ have been studied with neutron-diffraction measurements and self-consistent spin-polarized electronic-structure calculations. The diffraction results indicate for the Nd compound that N goes into two sites in two or more phases of varying fractional N occupations. For the Y compound N occupies only one site. Electronic-structure calculations for Y2Fe17 and Y2Fe17N3 give excellent results for site-dependent Fe moments, and, with spin-fluctuation theory, explain the large change in the Curie temperature on nitrogenation.

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