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Articles 61 - 90 of 97
Full-Text Articles in Engineering Physics
Effect Of Multi-Mode Vibration On Signature Estimation Using A Laser Vibration Sensor, Ngoya Pepela
Effect Of Multi-Mode Vibration On Signature Estimation Using A Laser Vibration Sensor, Ngoya Pepela
Theses and Dissertations
A laser vibration sensor (LVS) can be used to determine the vibrational spectrum of targets such as vehicles using heterodyne laser Doppler velocimetry. The vibrational spectra of exterior skin of vehicles are known to have characteristic resonances due to the physical structure driven by motor gears and other moving parts. Each particular class of vehicle has a unique vibrational spectrum. This research shows how a body vibrating in higher order modes has the opportunity to eliminate spectral content of the target's vibrational spectrum while using an LVS to perform spectrum estimation. This is due to roughly equal amounts of laser …
Time-Resolved Photoluminescence Of Inas/Gainsb Quantum Well Lasers, Michael R. Mckay
Time-Resolved Photoluminescence Of Inas/Gainsb Quantum Well Lasers, Michael R. Mckay
Theses and Dissertations
In the world of semiconductor photonic device fabrication, one important objective may be to extract as much light as possible from the device. In these devices, photons are created when electrons recombine with holes by transitioning from a high-energy state to a lower one. Unfortunately, electron-hole recombination does not always result in the formation of a photon. There are three basic types of recombination: the first results in the formation of a photon and is called radiative recombination; and the second and third, known as Shockley-Read-Hall and Auger recombination, result in the heating of the device and do not produce …
Single-Sided Noninvasive Inspection Of Multielement Sample Using Fan-Beam Multiplexed Compton Scatter Tomography, Matthew A. Lange
Single-Sided Noninvasive Inspection Of Multielement Sample Using Fan-Beam Multiplexed Compton Scatter Tomography, Matthew A. Lange
Theses and Dissertations
As aircraft age, corrosion forms upon unobservable surfaces, particularly at the junction of the sheet aluminum and the steel rivets used to attach the sheets to the airframe, degrading the aircraft s airworthiness. Previous research developed a noninvasive technique for the evaluation of the material composition of aluminum surfaces, utilizing the information encoded in the energy spectra of Compton-scattered gamma emissions. The spectra are gathered by a six-element, high purity germanium detector array. A, first principles, deterministic computer code is used to reconstruct a two-dimensional map of the electron density of aluminum samples. Previous efforts, to image pure aluminum samples, …
Spectral And Temporal Characterization Of High Temperature Events, William F. Bagby
Spectral And Temporal Characterization Of High Temperature Events, William F. Bagby
Theses and Dissertations
The remote observations of the temporal and spectral characteristics of the infrared (IR) emissions from exploding ordnance have been correlated with explosion conditions. A Bomem MR-154 Fourier Transform Interferometer with two detectors, InSb and HgCdTe, was used to record spectra in the 1.3 - 20 micrometers range. Data was collected at spectral resolutions of 16/cm and 4/cm, and temporal resolutions of 0.045 sand 0.123 5, respectively. The data files range in size from 900 Kilobytes to several Megabytes. These are reduced to 2-dimensional representations of temporal features that are less than 100 Kilobytes. The data analysis indicates the possibility of …
Use Of Quantum Mechanical Calculations To Investigate Small Silicon Carbide Clusters, Jean W. Henry
Use Of Quantum Mechanical Calculations To Investigate Small Silicon Carbide Clusters, Jean W. Henry
Theses and Dissertations
Density Functional Theory (DFT) method was employed to model silicon carbide small clusters. Comparing the DFT calculation results with experimental results that observed by using photoelectron spectroscopy (PES), DFT predicts the same structures that experiment observed. For electron affinity, DFT results are in good agreement with experimental results, the root mean square negative offset 0.1 eV found using medium size of basis set (cc-pVDZ+) calculation. DFT results for vibrational frequencies are in good agreement with experiment results; the root mean square error is 72.5 cm-1 wave number. 16 ground state structures of SimCn (m ≤ 4, …
Validation Of The Ionospheric Forecast Model (Ifm) Version 3, Michael D. Scott
Validation Of The Ionospheric Forecast Model (Ifm) Version 3, Michael D. Scott
Theses and Dissertations
The purpose of this research was to validate the Ionospheric Forecast Model (IFM) Version 3 to assess its suitability and usefulness as an operational tool. The Ionospheric forecast model is a first principles computer model designed to forecast the state of the global ionosphere to 24 hours. The scope was limited to a comparison of the F2 layer critical frequency (foF2) and peak electron density (hmF2) against observed ionosonde data. The model was run with global solar and geomagnetic indices and Information from Digitial Ionospheric Sounding System (DISS) observations as inputs. The DISS observations were input through the Parameterized Real-Time …
Temperature Determination In An I2 Gas Using Dopper-Limited Saturation Spectroscopy, James W. Myers Jr.
Temperature Determination In An I2 Gas Using Dopper-Limited Saturation Spectroscopy, James W. Myers Jr.
Theses and Dissertations
Saturation spectroscopy techniques were used to demonstrate the ability to make spatially resolved temperature measurements by exploiting the B3 ∏(O+u) - X1 Σ ;(O+g) electronic transition in molecular iodine (I2). Doppler-free saturation spectroscopy measurements resulted in hyperfine spectral profiles for the P(7O) 17-1 and P(53) 19-2 ro-vibrational transitions between the electronic levels. Doppler-limited saturation spectroscopy, an adaptation of the Doppler-free technique in which beams propagate in the same direction and cross within the gas sample, allowed for the measurement of spatially resolved Doppler profiles for the same transitions. The profiles were measured …
Application Of The Interaction Picture To Reactive Scattering In One Dimension, Michael J. Maclachlan
Application Of The Interaction Picture To Reactive Scattering In One Dimension, Michael J. Maclachlan
Theses and Dissertations
The interaction picture is used together with the channel-packet method in a new time-dependent approach to compute reactive scattering matrix elements. The channelpacket method enables the use of the interaction picture for computing reactive S-matrix elements by splitting the computational effort into two parts. First, asymptotic reactant and product wavepackets are individually propagated into the interaction region of the potential to form Moller states. The interaction picture, in contrast to the usual Schrödinger picture of quantum mechanics, is so constructed that a wavefunction that experiences no change in potential (that is, a free-particle wavefunction) remains always fixed, with no translation …
Implementation Of A Two-Dimensional Hydrodynamic Shock Code Based Upon The Weighted Average Flux Method, Mark P. Wittig
Implementation Of A Two-Dimensional Hydrodynamic Shock Code Based Upon The Weighted Average Flux Method, Mark P. Wittig
Theses and Dissertations
Numerical modeling of shock propagation and reflection is of interest to the Department of Defense (DoD). Propriety state-of-the-art codes based upon E. F. Toro's weighted average flux (WAF) method are being used to investigate complex shock reflection phenomena. Here we develop, test, and validate a one-dimensional hydrodynamic shock code. We apply WAF to Gudonov's first-order upwind method to achieve second-order accuracy. Oscillations, typical of second-order methods, are then removed using adaptive weight limiter functions based upon total variation diminishing (TVD) flux limiters. An adaptive Riemann solver routine is also implemented to improve computational efficiency. This one-dimensional code is then extended …
Space Charge Structure Of A Glow Discharge In The Presence Of A Longitudinal Inhomogeneity, Frank A. Tersigni
Space Charge Structure Of A Glow Discharge In The Presence Of A Longitudinal Inhomogeneity, Frank A. Tersigni
Theses and Dissertations
A survey of space charge structures arising due to inhomogeneities in glow discharges was conducted. Space charge structures associated with tube geometries, the cathode sheath, striations, and shockwaves were examined. Space charge effects on the Electron Energy Distribution Function (EEDF) were explored for a geometric inhomogeneity using an approximate nonlocal solution to the one dimensional Boltzmann equation after Godyak. The approximate solution partially captured qualitative aspects of space charge effects on the EEDF. Simplification of collisional effects and adaptation of an approximate electric field restricted quantitative comparisons with experimental data. It is recommended that any future analysis of space charge …
Band Structure Anisotropy In Semiconductor Quantum Wells, Steven J. Novotny
Band Structure Anisotropy In Semiconductor Quantum Wells, Steven J. Novotny
Theses and Dissertations
The focus of this research is an investigation of energy band anisotropy in simple quantum well structures. This anisotropy results from the asymmetry of the periodic potential within the crystal lattice. For sufficiently high doping levels, band structure anisotropy is expected to play an important role in the evaluation of the electronic and optical properties of the quantum well structures. The analysis uses a model based on a 6x6 Luttinger-Kohn k.p approach for bulk material valence band structure together with the Envelope Function Approximation. The model is used to analyze Si/SiGe, AlGaAs/GaAs, and GaAs/InGaAs quantum wells for the 001 and …
Optimization Of A Multilayer Photothermal Sensor For Infrared Spectroscopy, Janine O.J. Wiggins
Optimization Of A Multilayer Photothermal Sensor For Infrared Spectroscopy, Janine O.J. Wiggins
Theses and Dissertations
Tri-layer thermal diffusion modeling was applied to the optimization of a multi-layer reed sensor for use in a photothermal infrared detector. The multi-layer reed sensor deflects in response to increased temperature. Deflection, of angstroms or larger, is measured using an atomic force microscope. A newly developed thermal diffusion model for three layer reeds was combined with an existing two-layer cantilever model, in order to explore the effects of length, operating frequency, and layer thickness on signal to noise ratio. Model behavior is presented, and compared to laboratory results.
A Validation Of The Parameterized Real-Time Ionospheric Specification Model (Prism) Version 1.7b, Shawn D. Filby
A Validation Of The Parameterized Real-Time Ionospheric Specification Model (Prism) Version 1.7b, Shawn D. Filby
Theses and Dissertations
The most current version of the Parameterized Real-time Ionospheric Specification Model (PRISM), version 1.7b, was validated using Digital Ionospheric Sounding System (DISS) measurements of F2 layer critical frequency (foF2) and F2 peak electron density height (hmF2) as the "ground truth." PRISM was executed, first, with no real-time input parameter and, second, with Global Positioning System (GPS) Total Electron Content (TEC) measurements as the sole real-time parameter. Hourly values of hmF2 and foF2 over 123 days in 1994-1996 (solar minimum conditions) and covering three seasons (equinox, summer solstice, and winter solstice) were compared for Wallops Island, Virginia, and Point Arguello, California, …
Experimental Investigation Of Nonlinear Dynamics In Single Mode Semiconductor Laser Diodes With Phase Conjugate Feedback, Gordon T. Hengst
Experimental Investigation Of Nonlinear Dynamics In Single Mode Semiconductor Laser Diodes With Phase Conjugate Feedback, Gordon T. Hengst
Theses and Dissertations
The semiconductor laser diode offers a unique system to investigate nonlinear dynamics when optical feedback is applied. Although there is extensive research of laser diodes with optical feedback from normal dielectric mirrors, very little has been done experimentally to analyze the effects of degenerate phase conjugate feedback from a BaTiO3 crystal. This research experimentally investigated the dynamics of a single-mode laser diode with weak phase conjugate feedback using both the self-pumped and double phase conjugate geometries. The experimental results validated a mathematical model which numerically evaluates the Lang-Kobayahsi coupled differential equations. The model simulated the nonlinear behavior of a …
Unfolding The High Energy Electron Flux From Crres Fluxmeter Measurements, Brian D. Mckellar
Unfolding The High Energy Electron Flux From Crres Fluxmeter Measurements, Brian D. Mckellar
Theses and Dissertations
The Combined Release and Radiation Effects Satellite (CRRES) was launched on 25 July 1990 to collect measurements in the earth's radiation belts. One instrument, the High Energy Electron Fluxmeter (HEEF), measured the flux of electrons in 10 channels with energies between 1 MeV and 10 MeV. The channel sensitivities, Ri(E), have been calibrated and partially re-calibrated. We explore the errors introduced in unfolding the electron flux spectrum from the channel measurements and the propagation and growth of calibration and measurement errors. Using numerical experimentation, we fold the responses with known spectra to obtain simulated measurements, add random measurement and calibration …
Luminescence Study Of Ion-Implanted Gallium Nitride, Eric Silkowski
Luminescence Study Of Ion-Implanted Gallium Nitride, Eric Silkowski
Theses and Dissertations
Luminescence and absorption measurements were used to demonstrate the efficacy of ion implantation for introducing various classes of dopants into GaN. A wide range of implantation and annealing studies were performed with several dopant species (Ar, Zn, C, O, Si, Be, Mg, Nd, Er). Room temperature ion implantation was performed on MOCVD- and MBE-grown GaN samples at energies between 100 and 1150 keV with doses ranging from 1 x 1013 to 1 x 1015/cm-2. Conventional furnace annealing in flowing NH3 or N2 gas resulted in good implantation damage recovery at an annealing temperature …
A Monte Carlo Analysis Of Computerized Tomography, Karyl J. Davis
A Monte Carlo Analysis Of Computerized Tomography, Karyl J. Davis
Theses and Dissertations
The purpose of this thesis was to model a computerized tomography device (CT) using Monte Carlo methods to determine the scattered radiation spectrum inside and the dose outside the suite at Wright-Patterson Medical Center and at a generic suite to non-occupational personnel. This was driven by the recent inclusion of the most recent NCRP recommended dose limits into 10CFR20 of 50 mSv, occupational; 1 mSv; non-occupational continuous exposure; and 5 mSv, infrequent exposure. The rotating fan beam of the CT was modeled for MCNP, a general-purpose Monte Carlo n-particle transport modeL The CT target was a standard human phantom defined …
Photoluminescence And Electroluminescence Of Erbium And Neodymium Implanted Semiconductors, James R. Hunter
Photoluminescence And Electroluminescence Of Erbium And Neodymium Implanted Semiconductors, James R. Hunter
Theses and Dissertations
Low temperature photoluminescence (PL) and electroluminescence (EL) measurements were used to study the excitation of erbium- and neodymium-implanted GaAs and AlxGa1-xAs (x=0. 1, 0.3) pn-junctions. The rare-earth (RE) emissions were investigated as a function of ion dose, aluminum mole fraction, laser excitation power, and applied forward bias voltage for the implanted samples. Low temperature PL was also measured from Er doped silicon grown by the metalorganic chemical vapor-phase deposition (MOCVD) method using various growth parameters.. The MOCVD-grown Si samples were studied as a function of metalorganic source temperature, silane (SiH4) flow, growth time, and …
Optimal Wavefront Reconstruction For A Coherent Diffracted Field, William W. Arrasmith
Optimal Wavefront Reconstruction For A Coherent Diffracted Field, William W. Arrasmith
Theses and Dissertations
In sheared coherent beam interferometric imaging, an estimate of the average reflectivity profile of the object can be computed from measurements of point to point phase differences in the far field interference pattern and a suitable phase reconstruction technique. The phase difference information is encoded in the irradiance of three identical, shifted and superimposed speckled laser beam patterns. A minimum variance phase reconstruction technique is presented to estimate the phase of the field in the measurement plane from the phase differences and evaluate its performance. Prior knowledge of the phase covariance is used in the minimum variance reconstructor. Analytic calculations …
Multispectral Detection Of Ground Targets In Highly Correlated Backgrounds, Jason E. Thomas
Multispectral Detection Of Ground Targets In Highly Correlated Backgrounds, Jason E. Thomas
Theses and Dissertations
Multispectral detection methods attempt to discriminate targets in a dominant clutter background using multiple images of the same real-world scene taken in different narrow spectral bands in the infrared. Detection is possible due to the empirically observed phenomenon that the radiance of man-made objects, such as a tank or truck, often lies off the main spectral correlation axis of that of natural backgrounds. Radiometric measurements of several vehicles and a tree canopy background taken over three days in June. 1994 were used to examine the factors affecting multispectral detection. Results clearly showed that the processes which provide for higher spectral …
A Two-Dimensional Particle Simulation Of Parallel Plate Radio-Frequency (Rf) Glow Discharges, Eric J. Bennett
A Two-Dimensional Particle Simulation Of Parallel Plate Radio-Frequency (Rf) Glow Discharges, Eric J. Bennett
Theses and Dissertations
A two-dimensional model of parallel plate RF glow discharges was developed to study discharge phenomena important in plasma assisted processing of materials. The particle-in-cell method is used to calculate the trajectories of computer particles under the influence of both self and applied fields. Monte Carlo methods using the null collision technique are used to model collisions between charged particles and neutral gas atoms. Results of computer experiments are presented with special emphasis placed on ion motion in the sheath regions. Experimental results show some qualitative agreement with one-dimensional model results. Further work required to gain quantitative agreement is outlined.
Processability Of Polythiophene Thin Films By Ultraviolet Photo Bleaching, Derek D. Fletcher
Processability Of Polythiophene Thin Films By Ultraviolet Photo Bleaching, Derek D. Fletcher
Theses and Dissertations
Materials possessing strong ��(3) optical properties such as Polythiophene are sought for the production of optical switches. Polythiophene thin films produced by plasma enhanced CVD show a surface rms roughness of 10-15 angstroms over single square micron areas which is acceptable for wave guiding in the near IR. This research investigates permanently changing the optical properties of such a thin film by exposure to UV radiation (254 nm, 35 mW/cm2), known as photo bleaching, in hope of creating a refractive index boundary for use in total internal reflection. After 60 minutes exposure the refractive index shows …
Cation Production And Reactions Induced By Electron Impact On Tetraethoxysilane, Jeremy C. Holtgrave
Cation Production And Reactions Induced By Electron Impact On Tetraethoxysilane, Jeremy C. Holtgrave
Theses and Dissertations
The production of cations by electron impact on tetraethoxysilane (TEOS) is studied with a Fourier Transform mass spectrometer (FTMS). The operating principles of FTMS are reviewed and the experimental approach to the mass calibration and cross-section measurement is discussed. The cross-sections for total and partial ionization of Si(OC2H5)4 from threshold to 50 eV are measured. Also, the ion chemistry resulting from interactions between ions and neutral TEOS is examined.
Electronic-To-Vibrational (E → V) Energy Transfer From Br* To Co2 And Electronic-To-Vibrational (E → V) Laser Feasibility Studies, Steven M. Katapski
Electronic-To-Vibrational (E → V) Energy Transfer From Br* To Co2 And Electronic-To-Vibrational (E → V) Laser Feasibility Studies, Steven M. Katapski
Theses and Dissertations
The energy transfer mechanism from photo-excited bromine atoms (Br(2P½)) to C02 is investigated in a pulsed fluorescence experiment. An excimer-pumped pulsed dye laser operating at 480 nm is used to photolyze molecular bromine, resulting in the creation of one excited state bromine atom, Br(2P½), and one ground state atom, Br(2P3/2). The electronically- excited bromine atoms (referred to as Br*) collide with and excite vibrational modes in the ground electronic state of CO2. Measurements are made of the Br* lifetimes and associated quenching processes, and the …
Photoluminescence Study Of Si1-XGeX/Si And Si/Ge Strained Layer Superlattices, Todd D. Steiner
Photoluminescence Study Of Si1-XGeX/Si And Si/Ge Strained Layer Superlattices, Todd D. Steiner
Theses and Dissertations
In recent years, there has been interest in obtaining efficient photoluminescence (PL) and electroluminescence from Si1-xGex/Si or Si/Ge superlattices. In this dissertation, longperiod c/Si superlattices grown by molecular beam epitaxy at 500°C and annealed post-growth using rapid thermal annealing were investigated using photoluminescence (PL). The as-grown samples have broad PL bands from isoelectronic centers related to Ge complexes as well as sharp near-edge bound exciton lines. The broad PL band was found to be 120 meV below the band gap, after accounting for the effects of confinement of holes. Post-growth annealing resulted in a reduction in …
Shock Tube Simulation By The Smooth Particle Hydrodynamic (Sph) Method, Luke A. Lorang
Shock Tube Simulation By The Smooth Particle Hydrodynamic (Sph) Method, Luke A. Lorang
Theses and Dissertations
A smooth particle hydrodynamic code (SPHC) is evaluated for performing shock wave simulations by application to 1-D shock tube problems. Results of a shock tube test case with a compression ratio of 10 are compared against a Riemann shock tube problem and theoretical predictions of shock tube behavior to validate the SPH code. A Lagrangian hydrodynamic code is validated in a similar fashion. The resolution capabilities of both codes are compared using 100, 200 and 500 particles for SPHC and 100, 400 and 800 cells for the Lagrangian code. The SPH code exhibits a sharp spike in density at the …
Numerical Analysis Of Plasma Transport In Tandem Volume Magnetic Multicusp Ion Sources, Todd R. Vitko
Numerical Analysis Of Plasma Transport In Tandem Volume Magnetic Multicusp Ion Sources, Todd R. Vitko
Theses and Dissertations
A one-dimensional fluid model of plasma transport in tandem volume magnetic multicusp ion sources is explored. The model, the positive ion source code pos, by Glasser and Smith, calculates plasma density, drift velocity, electron temperature, and ion temperature in an ion source. The usefulness of the model is limited: (1) The plasma density trend runs opposite to experimental results, and electron temperatures are an order of magnitude higher than experimentally observed. (2) simplification of the reaction chemistry leads to a plasma balance between ionization and outflow instead of the correct balance between ionization and recombination. (3) Wall losses are …
An Analysis Of One-Dimensional Models Of The Insulating And Conducting Priz, Joseph W. Cook Iii
An Analysis Of One-Dimensional Models Of The Insulating And Conducting Priz, Joseph W. Cook Iii
Theses and Dissertations
This study attempted to analytically reproduce experimental and analytical data, on the single-insulated and conducting PRIZ, presented in 1987 and 1988 by the Soviet scientists Bliznetsov et al. Our onedimensional analytical models, derived from earlier Soviet reports, have consistently reproduced previously reported data on the PRIZ. In this study, the same codes were successfully used to analyze the effects of injection at high and low intensity exposures (30 µW/cm2 to 500 mW/cm2) Although the codes qualitatively duplicated the nature of the space charge distribution in the single-insulated and conducting PRIZ, our quantitative results were significantly different from …
Photoluminescence Spectroscopy Of Mbe Grown Gallium Antimonide, John A. Dorian
Photoluminescence Spectroscopy Of Mbe Grown Gallium Antimonide, John A. Dorian
Theses and Dissertations
Experimental data are presented for the energy levels of the native acceptor and bound exciton emissions of GaSb grown by MBE and studied with low temperature photoluminescence (PL) spectroscopy. The undoped GaSb emissions showed a donor-acceptor pair transition with an observed emission peak at 776 meV, a peak at 716 meV usually associated with the ionization level of the acceptor, and bound exciton transitions in the energy range from 796 to 804 meV. Other unidentified peaks were observed at about 805 meV. Temperature and power dependence studies of the acceptor peak confirmed donor-acceptor pair (DAP) behavior. A GaSb sample grown …
Low Temperature Photoluminescence Study Of Silicon-Germanium Alloy Superlattices, Maxwell M. Chi
Low Temperature Photoluminescence Study Of Silicon-Germanium Alloy Superlattices, Maxwell M. Chi
Theses and Dissertations
Low temperature photoluminescence studies were performed on nine silicon germanium alloy superlattice samples. The luminescence spectra showed sharp peaks in the 0.95 to 1.05 eV energy range, and a broad band located 0.05 to 0.12 eV below the alloy bandgap. The sharp peaks were identified as transitions associated with impurity bound excitons. The linewidth was about 10 times that in pure silicon; in addition, the peaks shifted to lower energy as the sample temperature was raised from 1.4 to 15 K. These features were attributed to effects of random distribution of atoms in the alloy. The broad band shifted to …