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Articles 33211 - 33240 of 36554

Full-Text Articles in Physical Sciences and Mathematics

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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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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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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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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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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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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33rd Rocky Mountain Conference On Applied Spectroscopy Jul 1991

33rd Rocky Mountain Conference On Applied Spectroscopy

Rocky Mountain Conference on Magnetic Resonance

Program and registration information for the 33rd annual meeting of the Rocky Mountain Conference on Applied Spectroscopy, sponsored by the Rocky Mountain Section of the Society for Applied Spectroscopy. Held in Denver, Colorado, July 28 - August 1, 1991.

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Three-Point Correlations In Driven Diffusive Systems With Ising Symmetry, Beate Schmittmann, R. K. P. Zia Jul 1991

Three-Point Correlations In Driven Diffusive Systems With Ising Symmetry, Beate Schmittmann, R. K. P. Zia

Beate Schmittmann

For equilibrium systems with Ising symmetry, the three-point correlation function is always zero above criticality. When a lattice-gas version of this system is driven to a nonequilibrium steady state, this correlation becomes nontrivial. Its dominant large-scale behavior is found to be a consequence of both the manifest breaking of Ising symmetry by the driving force and the more subtle violation of the fluctuation-dissipation theorem.

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Construction Of A Quasiconserved Quantity In The Henon-Heiles Problem Using A Single Set Of Variables, Paul Finkler, C. Edward Jones, Glenn A. Sowell Jul 1991

Construction Of A Quasiconserved Quantity In The Henon-Heiles Problem Using A Single Set Of Variables, Paul Finkler, C. Edward Jones, Glenn A. Sowell

Paul Finkler Papers

The problem of finding the coefficients of a simple series expansion for a quasiconserved quantity K for the Henon-Heiles Hamiltonian H using a single set of variables is solved. In the past, this type of approach has been problematic because the solution to the equations determining the coefficients in the expansion is not unique. As a result, the existence of a consistent expression for K to all orders had not previously been established. We show how to deal with this arbitrariness in the expansion coefficients for K in a consistent way. Due to this arbitrariness, we find a class of …

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Solid State Proton Spin Relaxation In Ethylbenzenes: Methyl Reorientation Barriers And Molecular Structure, Peter A. Beckmann, Laura Happersett, Antonia V. Herzog, William M. Tong Jul 1991

Solid State Proton Spin Relaxation In Ethylbenzenes: Methyl Reorientation Barriers And Molecular Structure, Peter A. Beckmann, Laura Happersett, Antonia V. Herzog, William M. Tong

Physics Faculty Research and Scholarship

We have investigated the dynamics of the ethyl groups and their constituent methyl groups in polycrystalline ethylbenzene (EB), 1,2-diethylbenzene (1,2-DEB), 1,3-DEB, and 1,4-DEB using the solid state proton spin relaxation (SSPSR) technique. The temperature and Larmor frequency dependence of the Zeeman spin-lattice relaxation rate is reported and interpreted in terms of the molecular dynamics. We determine that only the methyl groups are reorienting on the nuclear magnetic resonance time scale. The observed barrier of about 12 kJ/mol for methyl group reorientation in the solid samples of EB, 1,2-DEB, and 1,3-DEB is consistent with that of the isolated molecule, implying that …

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Target Dependence Of Angular Distributions For Near-Threshold (E,2E) Processes, Cheng Pan, Anthony F. Starace Jul 1991

Target Dependence Of Angular Distributions For Near-Threshold (E,2E) Processes, Cheng Pan, Anthony F. Starace

Anthony F. Starace Publications

Distorted-wave calculations of the triply differential cross sections for electron-impact ionization of H and He targets are presented for final-state electrons sharing 4-eV excess energy and leaving in opposite directions. The experimentally observed target dependence of the angular distributions is shown to stem essentially from short-range effects on the s-wave phase shifts of both incident and final-state continuum electrons.

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Study Of The Effect Of Relativistic Time Dilation On Cosmic Ray Muon Flux - Undergraduate Modern Physics Experiment, Nalini Easwar, Douglas A. Macintire Jul 1991

Study Of The Effect Of Relativistic Time Dilation On Cosmic Ray Muon Flux - Undergraduate Modern Physics Experiment, Nalini Easwar, Douglas A. Macintire

Physics: Faculty Publications

An experiment to study the effect of relativistic time dilation on secondary muon fluxes observed at different altitudes is described in this article. Muons, produced as secondary particles from the interaction of primary cosmic rays with the upper atmosphere, form a natural and abundant source of subatomic ``clocks'' moving at very high speeds. The measured muon flux on a mountain relative to that measured at sea level can be compared to predictions from calculations that take into account the relativistic time dilation in the muon frame. Situations under which such an experiment can be successfully performed are explored with a …

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Infrared Quenching And Thermal Recovery Of Thermally Stimulated Current Spectra In Gaas, Z-Q. Fang, David C. Look Jul 1991

Infrared Quenching And Thermal Recovery Of Thermally Stimulated Current Spectra In Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

Thermally stimulated current (TSC) spectra stimulated by infrared (hν≤1.12 eV) light at 90 K have been used to study the photoquenching and thermal recovery of several dominant TSC peaks in Ga‐rich, semi‐insulating GaAs. The filling‐pulse‐length dependence of the quenching for these traps, and the temperature dependence of their recovery are clearly identified with the ground‐to‐metastable state transition of EL2. The data are consistent with the traps having a direct association with EL2 or EL2∗ rather than an indirect association which could result from a change in the dominant‐free carriers as EL2 transforms to EL2∗. If so, they likely …

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Dielectronic Recombination On And Electron-Impact Excitation Of Heliumlike Argon, Rami M. Ali, Chander P. Bhalla, C. Lewis Cocke, Michael Schulz, Martin P. Stockli Jul 1991

Dielectronic Recombination On And Electron-Impact Excitation Of Heliumlike Argon, Rami M. Ali, Chander P. Bhalla, C. Lewis Cocke, Michael Schulz, Martin P. Stockli

Physics Faculty Research & Creative Works

We have measured cross sections for Δn=1 dielectronic recombination (DR) on He-like argon and found good agreement with theoretical calculations based on the Hartree-Fock atomic model. Experimental absolute cross sections were obtained by using the electron-energy dependence of yields of He-like and Li-like argon ions from the Kansas State University electron-beam ion source to measure the ratio of the DR cross section on He-like argon to the electron-impact-ionization cross section of Li-like argon and normalizing to the latter. The K x-ray emission spectra due to Δn=1,2 DR and n=1→2 electron-impact excitation of He-like argon were also observed with a Si(Li) …

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Near-Infrared Imaging Of Hydroxyl Wave Structure Over An Ocean Site At Low Latitudes, Michael J. Taylor, M. J. Hill Jul 1991

Near-Infrared Imaging Of Hydroxyl Wave Structure Over An Ocean Site At Low Latitudes, Michael J. Taylor, M. J. Hill

All Physics Faculty Publications

Coordinated observations of wave structure in the near infrared hydroxyl (OH) nightglow emission have been made from Maui, Hawaii using a suite of narrow angle and all‐sky TV cameras. Two sets of data were obtained, the first in conjunction with the ALOHA‐90 campaign and the second during the subsequent new moon period. Well formed, short period (<20 min) wave patterns of comparable morphology, dynamics and abundance to those regularly imaged from mid‐latitude mountain sites were detected on several occasions. Although the Hawaiian islands comprise several high volcanic peaks, the patterns were not consistent with gravity waves generated by the interaction of strong winds with the local island topography. This suggests that other mid‐latitude wave patterns may also not be of mountain origin. The wave patterns imaged during ALOHA‐90 were of significantly lower contrast than those detected later. This effect may be related to changes in the characteristics of the middle atmosphere that occur shortly after the spring equinox.

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Observations Of Short Period Mesospheric Wave Patterns: In Situ Or Tropospheric Wave Generation, Michael J. Taylor, R. Edwards Jul 1991

Observations Of Short Period Mesospheric Wave Patterns: In Situ Or Tropospheric Wave Generation, Michael J. Taylor, R. Edwards

All Physics Faculty Publications

Near infrared images showing wave structure in the hydroxyl (OH) nightglow emission have been obtained from Maui, Hawaii during the ALOHA‐90 campaign. Analysis of two nights during this campaign (25 and 31 March) indicate extensive, highly coherent, linear wave patterns of very short apparent period (∼5 and 10 min respectively). Both displays exhibited several features characteristic of the in situ breakdown of a large scale, long period, upper atmospheric wave disturbance. Data in support of this mechanism was found by other ALOHA instruments which detected concurrent long period (1–2 hour) mesospheric wave disturbances on both occasions. However, a tropospheric source …

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Enhanced Vibrational Broadening Of Core-Level Photoemission From The Surface Of Na(110), D. Mark Riffe, G. K. Wertheim, P. H. Citrin Jul 1991

Enhanced Vibrational Broadening Of Core-Level Photoemission From The Surface Of Na(110), D. Mark Riffe, G. K. Wertheim, P. H. Citrin

All Physics Faculty Publications

High-resolution temperature-dependent photoemission data from Na 2p core levels reveal substantially larger phonon broadening in the first atomic layer of Na(110) than in the bulk. We show that the enhanced width is due primarily to the excitation of relatively soft phonon modes perpendicular to the surface. Soft surface-phonon modes also account for previously reported but uninterpreted broadening of transition-metal surface-atom core levels.

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Coincident Imaging And Spectrometric Observations Of Zenith Oh Nightglow Structure, Michael J. Taylor, D. N. Turnbull, R. P. Lowe Jul 1991

Coincident Imaging And Spectrometric Observations Of Zenith Oh Nightglow Structure, Michael J. Taylor, D. N. Turnbull, R. P. Lowe

All Physics Faculty Publications

During the ALOHA‐90 campaign a novel comparative study was made between near infrared wave structure imaged in the zenith using a CCD camera and that detected at infrared wavelengths by a Fourier Transform Spectrometer. Coincident measurements were made briefly on several occasions and for an extended period on 31 March. The temporal variations imaged in the near infrared structure during this night almost completely matched those detected in the OH (3,1) band spectrometer data when similar viewing fields were compared. However, the image data also displayed small scale wave forms that were not resolved by the larger field instrument. These …

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Crystal Field Splitting And Charge Flow In The Buckled-Dimer Reconstruction Of Si(100)—2× 1, G. K. Wertheim, D. Mark Riffe, J. E. Rowe, P. H. Citrin Jul 1991

Crystal Field Splitting And Charge Flow In The Buckled-Dimer Reconstruction Of Si(100)—2× 1, G. K. Wertheim, D. Mark Riffe, J. E. Rowe, P. H. Citrin

All Physics Faculty Publications

The effect of the 2×1 reconstruction on the core-electron binding energies of the outermost Si(100) layers has been determined using high-resolution photoemission data. A previously unobserved 190-meV crystal-field splitting is resolved for the up-atoms of the asymmetric surface dimers, whose average core-level shift is -400 meV. The signal from the down-atoms is clearly identified and has a shift of +220 meV. These new findings indicate a charge flow of ∼0.05e from the subsurface to the surface layers, with a substantially larger difference of ∼0.34e between the up-atoms and down-atoms in the dimer.

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Interference Between Diffraction And Transmission In The Mie Extinction Efficiency, James A. Lock, Leiming Yang Jul 1991

Interference Between Diffraction And Transmission In The Mie Extinction Efficiency, James A. Lock, Leiming Yang

Physics Faculty Publications

We give simple analytic and numerical demonstrations showing that the interference structure in the Mie extinction efficiency of a sphere is caused by the interference of the light waves that are diffracted and transmitted in the near-forward direction.

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Angular Distribution Of Electrons Following Two-Photon Ionization Of The Ar Atom And Two-Photon Detachment Of The F- Ion, Cheng Pan, Anthony F. Starace Jul 1991

Angular Distribution Of Electrons Following Two-Photon Ionization Of The Ar Atom And Two-Photon Detachment Of The F- Ion, Cheng Pan, Anthony F. Starace

Anthony F. Starace Publications

Angular-distribution asymmetry parameters for photoelectrons produced by two-photon ionization of the Ar atom and two-photon detachment of the F- ion are calculated for photon energies below the thresholds for single-photon ionization and single-photon detachment, respectively. Effects of electron correlations are included by perturbative methods. Good agreement is obtained between our results and those of a recent experimental angular-distribution measurement of two-photon detachment of the F- ion at λ=532 nm [C. Blondel, M. Crance, C. Delsart, and A. Giraud (unpublished)].

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On Shock Capturing For Liquid And Gas Media, Tze Jang Chen Jul 1991

On Shock Capturing For Liquid And Gas Media, Tze Jang Chen

Mathematics & Statistics Theses & Dissertations

The numerical investigation of shock phenomena in gas or liquid media where a specifying relation for internal energy is absent poses special problems. Classically, for gas dynamics the usual procedure is to employ a splitting scheme to remove the source terms from the Euler equations, then up-wind biased shock capturing algorithms are built around the Riemann problem for the system which remains. However, in the case where the Euler equations are formulated in the term of total enthalpy, a technical difficulty associated with equation splitting forces a pressure time derivative to be treated as a source term. This makes it …

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Interpretation Issues In Force Microscopy, Nancy Burnham, Richard Colton, Hubert Pollock Jun 1991

Interpretation Issues In Force Microscopy, Nancy Burnham, Richard Colton, Hubert Pollock

Nancy A. Burnham

In this paper, we will discuss force microscopy (FM) and its potential for determining mechanical properties of thin films. We will introduce the basic principles of FM, and demonstrate how FM can be used to determine materials properties as well as image surface topography, both with nanonewton or sub‐nanonewton force resolution and sub‐nanometer position resolution. As FM is still a new field, not all of the questions concerning interpretation have been fully answered. We will elucidate four current issues that must be resolved before the full potential of FM can be realized. They are: (1) the role of water vapor …

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Investigation Of Real-Time Optical Scanning Holography, Bradley D. Duncan Jun 1991

Investigation Of Real-Time Optical Scanning Holography, Bradley D. Duncan

Electro-Optics and Photonics Faculty Publications

Real-time holographic recording using an optical heterodyne scanning technique was proposed by Poon in 1985. The first part of this dissertation provides a detailed theoretical treatment of the technique, based on a Gaussian beam analysis. Topics to be addressed include the derivations of the optical transfer function (OTF) and impulse response of the scanning holographic recording system, reconstructed image resolution and magnification, methods of carrier frequency hologram generation and experimental verification of the recording technique based on careful measurements of a hologram corresponding to a simple transmissive slit. Furthermore, computer simulations are presented pertaining to the incoherent nature of the …

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Ulf Waves In The Low‐Latitude Boundary Layer And Their Relationship To Magnetospheric Pulsations: A Multisatellite Observation, K. Takahashi, D. G. Sibeck, P. T. Newell, Harlan E. Spence Jun 1991

Ulf Waves In The Low‐Latitude Boundary Layer And Their Relationship To Magnetospheric Pulsations: A Multisatellite Observation, K. Takahashi, D. G. Sibeck, P. T. Newell, Harlan E. Spence

Physics & Astronomy

On April 30 (day 120), 1985, the magnetosphere was compressed at 0923 UT and the subsolar magnetopause remained near 7 REgeocentric for ∼2 hours, during which the four spacecraft Spacecraft Charging At High Altitude (SCATHA), GOES 5, GOES 6, and Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE were all in the magnetosphere on the morning side. SCATHA was in the low-latitude boundary layer (LLBL) in the second half of this period. The interplanetary magnetic field was inferred to be northward from the characteristics of precipitating particle fluxes as observed by the low-altitude satellite Defense Meteorological Satellite Program (DMSP) F7 and …

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On-Wafer Hall-Effect Measurement System, P. D. Mumford, David C. Look Jun 1991

On-Wafer Hall-Effect Measurement System, P. D. Mumford, David C. Look

Physics Faculty Publications

A novel system capable of making on‐wafer Hall‐effect measurements of a patterned wafer during the fabrication sequence has been developed. A flat, powerful rare‐earth magnet provides the magnetic field required. The wafer need only have van der Pauw patterns available for on‐wafer measurement capability. Measurement of room temperature Hall mobility can quickly and easily be obtained, making possible detailed study of carrier concentration and mobility variations during wafer fabrication.

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A Valence Quark Model, Tony A. Forest Jun 1991

A Valence Quark Model, Tony A. Forest

Masters Theses

A quark model of the baryons has been developed for calculating properties of nucleons which can be useful in nuclear interactions. A Hamiltonian, motivated by QCD, was diagonalized in a harmonic oscillator basis. In addition to predicting the proton charge radius and Delta-Nucleon mass difference, this work also yields a value of the axial vector coupling constant which compares well with experiment. Electric and magnetic form factors represent the major success of this model. The electromagnetic transitions predicted by this model compete well with other models but are not able to reproduce experimental results completely. The advantage of this model …

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Double Ionization Of Helium By Intermediate To High Velocity He+ Projectiles, Jun Forest Jun 1991

Double Ionization Of Helium By Intermediate To High Velocity He+ Projectiles, Jun Forest

Masters Theses

Ionization of helium target atoms by He+ projectiles colliding with helium at energies of 0.125 to 3.0 MeV/u is investigated. Cross sections for single and double ionization, and ratios o f double-to-single ionization are determined for each outgoing projectile charge state, and are compared to previous studies. For the lowest energies investigated, the ratios are consistent with the two-step mechanism in which the projectile interacts separately with each target electron. At the highest energies, the ratios reach nearly constant values indicating approach to the high velocity limit. At these high energies, however, the ratios are all higher than the …

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Resonant Electron Transfer And K-Shell Excitation Of FQ+ (Q=6,8) In Collisions With Neutral He And H2 Targets, Konstantinos E. Zaharakis Jun 1991

Resonant Electron Transfer And K-Shell Excitation Of FQ+ (Q=6,8) In Collisions With Neutral He And H2 Targets, Konstantinos E. Zaharakis

Masters Theses

Measurements of resonant transfer and excitation (RTE) were conducted for 16.5-38 MeV F6+ (Li-like) ions colliding with H2. In the RTE process electron capture and projectile excitation take place simultaneously due to the electronelectron interaction. In previous work, for the same collision system, at the Oak Ridge National Laboratory (Oak Ridge, TN) , the magnitude of the measured RTE cross sections was found to be nearly a factor of two smaller than theory. In order to investigate RTE in F6+ H2 collisions more completely, we undertook measurements at Western Michigan University using the EN tandem …

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Comparison Of Deep Centers In Semiinsulating Liquid-Encapsulated Czochralski And Vertical-Gradient Freeze Gaas, Z-Q. Fang, David C. Look Jun 1991

Comparison Of Deep Centers In Semiinsulating Liquid-Encapsulated Czochralski And Vertical-Gradient Freeze Gaas, Z-Q. Fang, David C. Look

Physics Faculty Publications

Three‐inch, semi‐insulating (SI) GaAs, grown by the vertical gradient freeze (VGF) technique, has been studied by IR absorption, temperature‐dependent dark current and Hall‐effect, thermally stimulated current (TSC), and photoinduced current transient spectroscopy and has been compared with undoped, SI GaAs, both As‐rich and Ga‐rich, grown by the high‐pressure liquid‐encapsulated Czochralski method. The results clearly indicate that (1) the VGF GaAs contains less EL2, which suggests a less As‐rich crystal stoichiometry; (2) in some VGF samples activation energies of 0.43 or 0.46 eV are deduced from temperature‐dependent carrier concentration or resistivity measurements, respectively, and (3) VGF samples often show a thermal …

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