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- Space-Charge Saturation and Current Limits in Cylindrical Drift Tubes and Planar Sheaths
- Space-charge effects play a dominant role in many areas of physics. In high-power microwave devices using high-current, relativistic electron beams, it places a limit on the amount of radiation a device can produce. Because the beam's space-charge can actually reflect a portion of the beam, the ability to accurately predict the amount of current a device can carry is needed. This current value is known as the space-charge limited current. Because of the mathematical difficulties, this limit is typically estimated from a one-dimensional theory. This work presents a two-dimensional theory for calculating an upper-bound for the space-charge limited current of relativistic electron beams propagating in grounded coaxial drift tubes. Applicable to annular beams of arbitrary radius and thickness, the theory includes the effect introduced by a finite-length drift tube of circular cross-section. Using Green's second identity, the need to solve Poisson's equation is transferred to solving a Sturm-Liouville eigenvalue problem, which is easily solved by elementary methods. In general, the resulting eigenvalue, which is required to estimate the limiting current, must be numerically determined. However, analytic expressions can be found for frequently encountered limiting cases. Space-charge effects also produce the fundamental collective behavior found in plasmas, especially in plasma sheaths. A plasma sheath is the transition region between a bulk plasma and an adjacent plasma-facing surface. The sheath controls the loss of particles from the plasma in order to maintain neutrality. Using a fully kinetic theory, the problem of a planar sheath with a single-minimum electric potential profile is investigated. Appropriate for single charge-state ions of arbitrary temperature, the theory includes the emission of warm electrons from the surface as well as a net current through the sheath and is compared to particle-in-cell simulations. Approximate expressions are developed for estimating the sheath potential as well as the transition to space-charge saturation. The case of a space-charge limited sheath is discussed and compared to the familiar Child-Langmuir law.
- Precision Atomic Spectroscopy with an Integrated Electro- Optic Modulator and DBR Diode Laser at 1083nm
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We have explored the use of recently developed high speed integrated electro optic modulators and DBR diode lasers as a tool for precision laser studies of atoms. In particular, we have developed a technique using a high speed modulator as a key element and applied it to the study of the fine structure of the 23P state of atomic helium. This state has been of long standing interest in atomic physics and its study has been the aim of several recent experiments using various precision techniques. We present our method and results, which will describe a new method for determining the fine structure constant, and lead to a precision test of atomic theory.
- Investigation of Selected Optically-Active Nanosystems Fashioned using Ion Implantation
- Opto-electronic semiconductor technology continues to grow at an accelerated pace, as the industry seeks to perfect devices such as light emitting diodes for purposes of optical processing and communication. A strive for greater efficiency with shrinking device dimensions, continually pushes the technology from both a design and materials aspect. Nanosystems such a quantum dots, also face new material engineering challenges as they enter the realm of quantum mechanics, with each system and material having markedly different electronic properties. Traditionally, the semiconductor industry has focused on materials such Group II-VI and III-V compounds as the basis material for future opto-electronic needs. Unfortunately, these material systems can be expensive and have difficulties integrating into current Si-based technology. The industry is reluctant to leave silicon due in part to silicon's high quality oxide, and the enormous amount of research invested into silicon based circuit fabrication. Although recently materials such as GaN are starting to dominate the electro-optical industry since a Si-based substitute has not been found. The purpose of the dissertation was to examine several promising systems that could be easily integrated into current Si-based technology and also be produced using simple inexpensive fabrication techniques such ion implantation. The development of optically active nano-sized precipitates in silica to form the active layer of an opto-electronic device was achieved with ion implantation and thermal annealing. Three material systems were investigated. These systems consisted of carbon, silicon and metal silicide based nanocrystals. The physical morphology and electronic properties were monitored using a variety of material characterization techniques. Rutherford backscattering/channeling were used to monitor elemental concentrations, photoluminescence was used to monitor the opto-electronic properties and transmission electron microscopy was used to study the intricate morphology of individual precipitates. The electronic properties and the morphology were studied as a function of implant dose, anneal times and anneal temperatures.
- Neutron Transmutation and Hydrogenation Study of Hg₁₋xCdxTe
- Anomalous Hall behavior of HgCdTe refers to a "double cross-over" feature of the Hall coefficient in p-type material, or a peak in the Hall mobility or Hall coefficient in n-type material. A magnetoconductivity tensor approach was utilized to identify presence of two electrons contributing to the conduction as well as transport properties of each electron in the material. The two electron model for the mobility shows that the anomalous Hall behavior results from the competition of two electrons, one in the energy gap graded region near the CdZnTe/HgCdTe interface with large band gap and the other in the bulk of the LPE film with narrow band gap. Hg0.78Cd0.22Te samples grown by LPE on CdZnTe(111B)-oriented substrates were exposed to various doses of thermal neutrons (~1.7 x 1016 - 1.25 x 1017 /cm2) and subsequently annealed at ~220oC for ~24h in Hg saturated vapor to recover damage and reduce the presence of Hg vacancies. Extensive Magnetotransport measurements were performed on these samples. SIMS profile for impurities produced by neutron irradiation was also obtained. The purpose for this study is to investigate the influence of neutron irradiation on this material as a basis for further study on HgCdTe74Se. The result shows that total mobility is observed to decrease with increased neutron dose and can be fitted by including a mobility inverse proportional to neutron dose. Electron introduction rate of thermal neutron is much smaller than that of fission neutrons. Total recovering of the material is suggested to have longer time annealing. Using Kane's model, we also fitted carrier concentration change at low temperature by introducing a donor level with activation energy changing with temperature. Results on Se diffusion in liquid phase epitaxy (LPE) grown HgCdTe epilayers is reported. The LPE Hg0.78Cd0.22Te samples were implanted with Se of 2.0×1014/cm2 at 100keV and annealed at 350-450oC in mercury saturated vapor. Secondary ions mass spectrometry (SIMS) profiles were obtained for each sample. From a Gaussian fit we find that the Se diffusion coefficient DSe is about one to two orders of magnitude smaller than that of arsenic. The as-implanted Se distribution is taken into account in case of small diffusion length in Gaussian fitting. Assuming a Te vacancy based mechanism, the Arrhenius relationship yields an activation energy 1.84eV. Dislocations introduced in HgCdTe materials result in two energy levels, where one is a donor and one is an acceptor. Hydrogenation treatment can effectively neutralize these dislocation defect levels. Both experimental results and theoretical calculation show that the mobility due to dislocation scattering remains constant in the low temperature range (<77K), and increases with temperature between 77K and 150K. Dislocation scattering has little effect on electrical transport properties of HgCdTe with an EPD lower than 107/cm2. Dislocations may have little effect on carrier concentration for semiconductor material with zinc blende structure due to self compensation.
- Computational studies of selected ruthenium catalysis reactions.
- Computational techniques were employed to investigate pathways that would improve the properties and characteristics of transition metal (i.e., ruthenium) catalysts, and to explore their mechanisms. The studied catalytic pathways are particularly relevant to catalytic hydroarylation of olefins. These processes involved the +2 to +3 oxidation of ruthenium and its effect on ruthenium-carbon bond strengths, carbon-hydrogen bond activation by 1,2-addition/reductive elimination pathways appropriate to catalytic hydrogen/deuterium exchange, and the possible intermediacy of highly coordinatively unsaturated (e.g., 14-electron) ruthenium complexes in catalysis. The calculations indicate a significant decrease in the Ru-CH3 homolytic bond dissociation enthalpy for the oxidation of TpRu(CO)(NCMe)(Me) to its RuIII cation through both reactant destabilization and product stabilization. This oxidation can thus lead to the olefin polymerization observed by Gunnoe and coworkers, since weak RuIII-C bonds would afford quick access to alkyl radical species. Calculations support the experimental proposal of a mechanism for catalytic hydrogen/deuterium exchange by a RuII-OH catalyst. Furthermore, calculational investigations reveal a probable pathway for the activation of C-H bonds that involves phosphine loss, 1,2-addition to the Ru-OH bond and then reversal of these steps with deuterium to incorporate it into the substrate. The presented results offer the indication for the net addition of aromatic C-H bonds across a RuII-OH bond in a process that although thermodynamically unfavorable is kinetically accessible. Calculations support experimental proposals as to the possibility of binding of weakly coordinating ligands such as dinitrogen, methylene chloride and fluorobenzene to the "14-electron" complex [(PCP)Ru(CO)]+ in preference to the formation of agostic Ru-H-C interactions. Reactions of [(PCP)Ru(CO)(1-ClCH2Cl)][BAr'4] with N2CHPh or phenylacetylene yielded conversions that are exothermic to both terminal carbenes and vinylidenes, respectively, and then bridging isomers of these by C-C bond formation resulting from insertion into the Ru-Cipso bond of the phenyl ring of PCP. The QM/MM and DFT calculations on full complexes [(PCP)(CO)Ru=(C)0,1=CHPh]+ and on small models [(PCP')(CO)Ru=(C)0,1=CH2]+, respectively, offered data supportive of the thermodynamic feasibility of the suggested experimental mechanisms and their proposed intermediates.
- Anderson Localization in Two-Channel Wires with Correlated Disorder: DNA as an Application
- This research studied the Anderson localization of electrons in two-channel wires with correlated disorder and in DNA molecules. It involved an analytical calculation part where the formula for the inverse localization length for electron states in a two-channel wire is derived. It also involved a computational part where the localization length is calculated for some DNA molecules. Electron localization in two-channel wires with correlated disorder was studied using a single-electron tight-binding model. Calculations were within second-order Born-approximation to second-order in disorder parameters. An analytical expression for localization length as a functional of correlations in potentials was found. Anderson localization in DNA molecules were studied in single-channel wire and two-channel models for electron transport in DNA. In both of the models, some DNA sequences exhibited delocalized electron states in their energy spectrum. Studies with two-channel wire model for DNA yielded important link between electron localization properties and genetic information.
- Microstructure and Electronic Structures of Er-Doped Si Nano-particles Synthesized by Vapor Phase Pyrolysis
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Si nanoparticles are new prospective optoelectronic materials. Unlike bulk Si cry-stals, Si nanoparticles display intriguing room-temperature photoluminescence. A major challenge in the fabrication of Si nanoparticles is the control of their size distribution. The rare-earth element Er has unique photo emission properties, including low pumping power, and a temperature independent, sharp spectrum. The emission wavelength matches the transmission window of optical fibers used in the telecommunications industry. Therefore, the study of Er-doped Si nanoparticles may have practical significance. The goals of the research described in this dissertation are to investigate vapor phase pyrolysis methods and to characterize the microstructure and associated defects, particles size distributions and photoluminescence efficiencies of doped and undoped Si nanoparticles using analytical transmission electron microscopy, high resolution electron microscopy, and optical spectroscopy. Er-doped and undoped Si nanoparticles were synthesized via vapor-phase pyrolysis of disilane at Texas Christian University. To achieve monodisperse size distributions, a process with fast nucleation and slow growth was employed. Disilane was diluted to 0.48% with helium. A horizontal pyrolysis oven was maintained at a temperature of 1000 °C. The oven length was varied from 1.5 cm to 6.0 cm to investigate the influence of oven length on the properties of the nanoparticles. The Si nanoparticles were collected in ethylene-glycol. The doped and undoped Si nanoparticles have a Si diamond cubic crystal structure. Neither Er precipitation, Er oxides or Er silicides were detected in any of the samples. The Er dopant concentration was about 2 atom% for doped samples from the 3.0 and 6.0 cm ovens as determined by quantitative analysis using X-ray energy dispersive spectroscopy. The average Si nanoparticle size increases from 11.3 to 15.2 nm in the doped samples and from 11.1 to 15.7 nm in the undoped samples as the oven length increases from 1.5 to 6.0 cm. HREM data show that average Si nanocrystallite size varies from 6.4 to 3.3 to 5.9 nm in the doped samples, and from 7.5 to 12.2 nm in the undoped samples as the oven length increases. Room-temperature Er photoluminescence has been detected near 1.54 :m from all doped samples. Saturation of the Er photoluminescence intensity at large emission power and the monotonic decrease of the intensity as a function of the emission wavelength in the doped sample from the 3.0 cm oven suggest that a carrier-mediated energy transfer process occurs in the Er-doped Si nanoparticles. It is the first time to successfully fabricate and investigate Er-doped Si nanoparticles.
- Energy Distribution of Sputtered Neutral Atoms from a Multilayer Target
- Energy distribution measurements of sputtered neutral particles contribute to the general knowledge of sputtering, a common technique for surface analysis. In this work emphasis was placed on the measurement of energy distribution of sputtered neutral atoms from different depths. The liquid Ga-In eutectic alloy as a sample target for this study was ideal due to an extreme concentration ratio gradient between the top two monolayers. In pursuing this study, the method of sputter-initiated resonance ionization spectroscopy (SIRIS) was utilized. SIRIS employs a pulsed ion beam to initiate sputtering and tunable dye lasers for resonance ionization. Observation of the energy distribution was achieved with a position-sensitive detector. The principle behind the detector's energy resolution is time of flight (TOF) spectroscopy. For this specific detector, programmed time intervals between the sputtering pulse at the target and the ionizing laser pulse provided information leading to the energy distribution of the secondary neutral particles. This experiment contributes data for energy distributions of sputtered neutral particles to the experimental database, required by theoretical models and computer simulations for the sputtering phenomenon.
- EEG, Alpha Waves and Coherence
- This thesis addresses some theoretical issues generated by the results of recent analysis of EEG time series proving the brain dynamics are driven by abrupt changes making them depart from the ordinary Poisson condition. These changes are renewal, unpredictable and non-ergodic. We refer to them as crucial events. How is it possible that this form of randomness be compatible with the generation of waves, for instance alpha waves, whose observation seems to suggest the opposite view the brain is characterized by surprisingly extended coherence? To shed light into this apparently irretrievable contradiction we propose a model based on a generalized form of Langevin equation under the influence of a periodic stimulus. We assume that there exist two different forms of time, a subjective form compatible with Poisson statistical physical and an objective form that is accessible to experimental observation. The transition from the former to the latter form is determined by the brain dynamics interpreted as emerging from the cooperative interaction among many units that, in the absence of cooperation would generate Poisson fluctuations. We call natural time the brain internal time and we make the assumption that in the natural time representation the time evolution of the EEG variable y(t) is determined by a Langevin equation perturbed by a periodic process that in this time representation is hardly distinguishable from an erratic process. We show that the representation of this random process in the experimental time scale is characterized by a surprisingly extended coherence. We show that this model generates a sequence of damped oscillations with a time behavior that is remarkably similar to that derived from the analysis of real EEG's. The main result of this research work is that the existence of crucial events is not incompatible with the alpha wave coherence. In addition to this important result, we find another result that may help our group, or any other research group working on the analysis of brain's dynamics, to prove or to disprove the existence of crucial events. We study the diffusion process generated by fluctuations emerging from the same model after filtering out the alpha coherence, and we study the recursion to the origin. We study the survival probability of this process, namely the probability that up to a given time no re-crossing of the origin occurs. We find that this is an inverse power law with a power that depends on whether or not crucial events exist.
- Growing carbon nanotubes by chemical vapor deposition technique.
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Carbon nanotubes were synthesized in the laboratory using chemical vapor deposition at different methane concentration. I found that a methane concentration of 4 sccm was ideal for well recognizable carbon nanotubes. A higher concentration led to fewer nanotube growth and silicon carbide structure. Coating the sample first with Fe(NO3)3 created a catalyst base on the substrate for the nanotube to adhere and grow on.
- Charge Collection Studies on Integrated Circuit Test Structures using Heavy-Ion Microbeams and MEDICI Simulation Calculations
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Ion induced charge collection dynamics within Integrated Circuits (ICs) is important due to the presence of ionizing radiation in the IC environment. As the charge signals defining data states are reduced by voltage and area scaling, the semiconductor device will naturally have a higher susceptibility to ionizing radiation induced effects. The ionizing radiation can lead to the undesired generation and migration of charge within an IC. This can alter, for example, the memory state of a bit, and thereby produce what is called a "soft" error, or Single Event Upset (SEU). Therefore, the response of ICs to natural radiation is of great concern for the reliability of future devices. Immunity to soft errors is listed as a requirement in the 1997 National Technology Roadmap for Semiconductors prepared by the Semiconductor Industry Association in the United States. To design more robust devices, it is essential to create and test accurate models of induced charge collection and transport in semiconductor devices. A heavy ion microbeam produced by an accelerator is an ideal tool to study charge collection processes in ICs and to locate the weak nodes and structures for improvement through hardening design. In this dissertation, the Ion Beam Induced Charge Collection (IBICC) technique is utilized to simulate recoil effects of ions in ICs. These silicon or light ion recoils are usually produced by the elastic scattering or inelastic reactions between cosmic neutrons or protons and the lattice atoms in ICs. Specially designed test structures were experimentally studied, using microbeams produced at Sandia National Laboratories. A new technique, Diffusion Time Resolved IBICC, is first proposed in this work to measure the average arrival time of the diffused charge, which can be related to the first moment (or the average time) of the arrival carrier density at the junction. A 2D device simulation tool, the MEDICI code, and heavy-ion microbeams are used to calculate and measure charge collection and relative arrival time on stripe-like test junctions. The MEDICI simulation is in qualitative and sometimes even quantitative agreement with the microbeam measurements. The amount of charge collection and the magnitude of average arrival time for diffused charge collection can be crucial to understanding and mitigating radiation induced circuit malfunctions during normal IC operations.
- Nonlinear UV Laser Build-up Cavity: An Efficient Design
- Using the concept of the build-up cavity for second harmonic generation to produce 243nm laser light, an innovative cavity is theoretically explored using a 15mm length CLBO crystal. In order to limit the losses of the cavity, the number of effective optical surfaces is kept to only four and the use of a MgF2 crystal is adopted to separate the harmonic and fundamental laser beam from each other. The cavity is shown to have an expected round trip loss of five tenths of a percent or better, resulting in a conversion efficiency greater than 65%.
- Application of the finite element method to some simple systems in one and two dimensions.
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The finite element method (FEM) is reviewed and applied to the one-dimensional eigensystems of the isotropic harmonic oscillator, finite well, infinite well and radial hydrogen atom, and the two-dimensional eigensystems of the isotropic harmonic oscillator and the propagational modes of sound in a rectangular cavity. Computer codes that I developed were introduced and utilized to find accurate results for the FEM eigensolutions. One of the computer codes was modified and applied to the one-dimensional unbound quantum mechanical system of a square barrier potential and also provided accurate results.
- Precision measurements of the hyperfine structure in the 23P state of 3He.
- The unusually large hyperfine structure splittings in the 23P state of the 3He isotope is measured using electro-optic techniques with high precision laser spectroscopy. Originally designed to probe the fine structure of the 4He atom, this experimental setup along with special modifications I implemented to resolve certain 3He related issues has made possible new high precision hyperfine structure measurements. Discussed are the details of the experimental setup and the modifications, including in depth information necessary to consider while performing these measurements. The results of these hyperfine structure measurements give an order of magnitude improvement in precision over the best previously reported values.
- Decoherence, Master Equation for Open Quantum Systems, and the Subordination Theory
- This thesis addresses the problem of a form of anomalous decoherence that sheds light into the spectroscopy of blinking quantum dots. The system studied is a two-state system, interacting with an external environment that has the effect of establishing an interaction between the two states, via a coherence generating coupling, called inphasing. The collisions with the environment produce also decoherence, named dephasing. Decoherence is interpreted as the entanglement of the coherent superposition of these two states with the environment. The joint action of inphasing and dephasing generates a Markov master equation statistically equivalent to a random walker jumping from one state to the other. This model can be used to describe intermittent fluorescence, as a sequence of "light on" and "light off" states. The experiments on blinking quantum dots indicate that the sojourn times are distributed with an inverse power law. Thus, a proposal to turn the model for Poisson fluorescence intermittency into a model for non-Poisson fluorescence intermittency is made. The collision-like interaction of the two-state system with the environment is assumed to takes place at random times rather than at regular times. The time distance between one collision and the next is given by a distribution, called the subordination distribution. If the subordination distribution is exponential, a sequence of collisions yielding no persistence is turned into a sequence of "light on" and "light off" states with significant persistence. If the subordination function is an inverse power law the sequel of "light on" and "light off" states becomes equivalent to the experimental sequences. Different conditions are considered, ranging from predominant inphasing to predominant dephasing. When dephasing is predominant the sequel of "light on" and "light off" states in the time asymptotic limit becomes an inverse power law. If the predominant dephasing involves a time scale much larger than the minimum time scale accessible to the experimental observation, thereby generating persistence, the resulting distribution becomes a Mittag-Leffler function. If dephasing is predominant, in addition to the inverse power law distribution of "light off" and "light on" time duration, a strong correlation between "light on" and "light off" state is predicted.
- A Reverberation Time Meter
- This thesis describes the construction of an apparatus to measure reverberation time.
- Studying Interactions of Gas Molecules with Nanomaterials Loaded in a Microwave Resonant Cavity
- A resonant cavity operating in TE011 mode was used to study the adsorption response of single walled carbon nanotubes (SWCNTs) and other nanomaterials for different types of gas molecules. The range of the frequency signal as a probe was chosen as geometry dependent range between 9.1 -9.8 GHz. A highly specific range can be studied for further experiments dependent on the type of molecule being investigated. It was found that for different pressures of gases and for different types of nanomaterials, there was a different response in the shifts of the probe signal for each cycle of gassing and degassing of the cavity. This dissertation suggests that microwave spectroscopy of a complex medium of gases and carbon nanotubes can be used as a highly sensitive technique to determine the complex dielectric response of different polar as well as non-polar gases when subjected to intense electromagnetic fields within the cavity. Also, as part of the experimental work, a range of other micro-porous materials was tested using the residual gas analysis (RGA) technique to determine their intrinsic absorption/adsorption characteristics when under an ultra-high vacuum environment. The scientific results obtained from this investigation, led to the development of a chemical biological sensor prototype. The method proposed is to develop operational sensors to detect toxin gases for homeland security applications and also develop sniffers to detect toxin drugs for law enforcement agency personnel.
- Ion Beam Synthesis of Carbon Assisted Nanosystems in Silicon Based Substrates
- The systematic study of the formation of β-SiC formed by low energy carbon ion (C-)implantation into Si followed by high temperature annealing is presented. The research is performed to explore the optimal annealing conditions. The formation of crystalline β-SiC is clearly observed in the sample annealed at 1100 °C for a period of 1 hr. Quantitative analysis is performed in the formation of β-SiC by the process of implantation of different carbon ion fluences of 1×1017, 2×1017, 5×1017, and 8×1017 atoms /cm2 at an ion energy of 65 keV into Si. It is observed that the average size of β-SiC crystals decreased and the amount of β-SiC crystals increased with the increase in the implanted fluences when the samples were annealed at 1100°C for 1 hr. However, it is observed that the amount of β-SiC linearly increased with the implanted fluences up to 5×1017 atoms /cm2. Above this fluence the amount of β-SiC appears to saturate. The stability of graphitic C-C bonds at 1100°C limits the growth of SiC precipitates in the sample implanted at a fluence of 8×1017 atoms /cm2 which results in the saturation behavior of SiC formation in the present study. Secondly, the carbon cluster formation process in silica and the characterization of formed clusters is presented. Silicon dioxide layers ~500 nm thick are thermally grown on a Si (100) wafer. The SiO2 layers are then implanted with 70 keV carbon ions at a fluence of 5×1017 atoms/cm2. The implanted samples are annealed 1100 °C for different time periods of 10 min., 30 min., 60 min., 90 min., and 120 min., in the mixture of argon and hydrogen gas (96 % Ar + 4% hydrogen). Photoluminescence spectroscopy reveals UV to visible emission from the samples. A detail mechanism of the photoluminescence and its possible origin is discussed by correlating the structural and optical properties of the samples. Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy are used to characterize the samples.
- Thorium and Uranium M-shell X-ray Production Cross Sections for 0.4 – 4.0 MeV Protons, 0.4 - 6.0 MeV Helium Ions, 4.5 – 11.3 MeV Carbon Ions, and 4.5 – 13.5 MeV Oxygen Ions.
- The M-shell x-ray production cross section for thorium and uranium have been determined for protons of energy 0.4 - 4.0 MeV, helium ions of energy 0.4 - 6.0 MeV, carbon ions of energy 4.5 - 11.3 MeV and oxygen ions of energy 4.5 - 13.5 MeV. The total cross sections and the cross sections for individual x-ray peaks in the spectrum, consisting of the following transitions Mz (M4-N2, M5-N3, M4-N3), Ma (M5-N6,7), Mb (M4-N6, M5-O3, M4- O2), and Mg (M4-O3, M5-P3, M3-N4, M3-N5), were compared to the theoretical values determined from the PWBA + OBKN and ECUSAR. The theoretical values for the carbon and oxygen ions were also modified to take into account the effects of multiple ionizations of the target atom by the heavier ions. It is shown that the results of the ECUSAR theory tend to provide better agreement with the experimental data.
- Dipole Moments of Diphenyl Compounds with Conjugated Double Bonds
- This thesis is a continuation of a study of molecular moments begun by Joseph T. Fielder. In his paper he discussed the theory and the equipment necessary for such a study. It is the purpose of this paper to set forth modifications of his equipment, to present data obtained with this modified equipment, and to interpret this data.
- A Determination of the Fine Structure Constant Using Precision Measurements of Helium Fine Structure
- Spectroscopic measurements of the helium atom are performed to high precision using an atomic beam apparatus and electro-optic laser techniques. These measurements, in addition to serving as a test of helium theory, also provide a new determination of the fine structure constant α. An apparatus was designed and built to overcome limitations encountered in a previous experiment. Not only did this allow an improved level of precision but also enabled new consistency checks, including an extremely useful measurement in 3He. I discuss the details of the experimental setup along with the major changes and improvements. A new value for the J = 0 to 2 fine structure interval in the 23P state of 4He is measured to be 31 908 131.25(30) kHz. The 300 Hz precision of this result represents an improvement over previous results by more than a factor of three. Combined with the latest theoretical calculations, this yields a new determination of α with better than 5 ppb uncertainty, α-1 = 137.035 999 55(64).
- Theoretical Study of Second Harmonic Generation of a Blue Laser at 486 nm Using a BBO Crystal in a Standing Wave Buildup Cavity
- For a spectroscopy purpose, we are interested in producing continuous wave (CW) UV laser light at 243 nm with at least 2 mW power. The theory of nonlinear optics suggests that we should be able to produce a desired 2.9 mW of 243 nm light by second harmonic generation (SHG) from a 50 mW blue laser at 486 nm using a BBO crystal in a build up cavity. The most important physical parameters are calculated. A 10 mm Brewster cut BBO crystal can provide phase matching conditions for coupling two ordinary photons at 486 nm and make a secondary beam at 243 nm. The single pass conversion efficiency is calculated not to be enough to generate 2.9 mW of SH light. My investigation shows that a standing wave build up cavity can provide a buildup factor of 94 and an overall conversion efficiency of 5.9% if one use an input coupler mirror with 1.1% transmission at 486 nm.
- The Effect of Average Grain Size on Polycrystalline Diamond Films
- The work function of hydrogen-terminated, polycrystalline diamond was studied using ultraviolet photoelectron spectroscopy. Polycrystalline diamond films were deposited onto molybdenum substrates by electrophoresis for grain sizes ranging from 0.3 to 108 microns. The work function and electron affinity were measured using 21.2 eV photons from a helium plasma source. The films were characterized by x-ray photoelectron spectroscopy to determine elemental composition and the sp2/sp3 carbon fraction. The percentage of (111) diamond was determined by x-ray diffraction, and scanning electron microscopy was performed to determine average grain size. The measured work function has a maximum of 5.1 eV at 0.3 microns, and decreases to 3.2 eV at approximately 4 microns. Then the work function increases with increasing grain size to 4.0 eV at 15 microns and then asymptotically approaches the 4.8 eV work function of single crystal diamond at 108 microns. These results are consistent with a 3-component model in which the work function is controlled by single-crystal (111) diamond at larger grain sizes, graphitic carbon at smaller grain sizes, and by the electron affinity for the intervening grain sizes.
- Carbon nanotube/microwave interactions and applications to hydrogen fuel cells.
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One of the leading problems that will be carried into the 21st century is that of alternative fuels to get our planet away from the consumption of fossil fuels. There has been a growing interest in the use of nanotechnology to somehow aid in this progression. There are several unanswered questions in how to do this. It is known that carbon nanotubes will store hydrogen but it is unclear how to increase that storage capacity and how to remove this hydrogen fuel once stored. This document offers some answers to these questions. It is possible to implant more hydrogen in a nanotube sample using a technique of ion implantation at energy levels ~50keV and below. This, accompanied with the rapid removal of that stored hydrogen through the application of a microwave field, proves to be one promising avenue to solve these two unanswered questions.
- Dipole Moments of Olefenic Diesters
- It is the purpose of this paper to present experimental data for the determination of the dielectric constant and the dipole moments for a series of olefenic diesters of the cis and trans configurations.
- A Deuterium-Deuterium Type Neutron Source
- In view of the advantages of its type, the decision to construct a neutron source of the particle accelerator type was made. The purpose of this thesis is to survey the problems encountered in the construction of the source.
- High Efficiency High Power Blue Laser by Resonant Doubling in PPKTP
- I developed a high power blue laser for use in scientific and technical applications (eg. precision spectroscopy, semiconductor inspection, flow cytometry, etc). It is linearly polarized, single longitudinal and single transverse mode, and a convenient fiber coupled continuous wave (cw) laser source. My technique employs external cavity frequency doubling and provides better power and beam quality than commercially available blue diode lasers. I use a fiber Bragg grating (FBG) stabilized infrared (IR) semiconductor laser source with a polarization maintaining (PM) fiber coupled output. Using a custom made optical and mechanical design this output is coupled with a mode matching efficiency of 96% into the doubling cavity. With this carefully designed and optimized cavity, measurements were carried out at various fundamental input powers. A net efficie ncy of 81 % with an output power of 680 mW at 486 nm was obtained using 840 mW of IR input. Also I report an 87.5 % net efficiency in coupling of blue light from servo locked cavity into a single mode PM fiber. Thus I have demonstrated a total fiber to fiber efficiency of 71% can be achieved in our approach using periodically poled potassium titanyl phosphate (PPKTP). To obtain these results, all losses in the system were carefully studied and minimized.
- Temporal Properties Of Dynamic Processes On Complex Networks
- Many social, biological and technological systems can be viewed as complex networks with a large number of interacting components. However despite recent advancements in network theory, a satisfactory description of dynamic processes arising in such cooperative systems is a subject of ongoing research. In this dissertation the emergence of dynamical complexity in networks of interacting stochastic oscillators is investigated. In particular I demonstrate that networks of two and three state stochastic oscillators present a second-order phase transition with respect to the strength of coupling between individual units. I show that at the critical point fluctuations of the global order parameter are characterized by an inverse-power law distribution and I assess their renewal properties. Additionally, I study the effect that different types of perturbation have on dynamical properties of the model. I discuss the relevance of those observations for the transmission of information between complex systems.
- A Continuously Sensitive Cloud Chamber
- A continuous cloud chamber would be a valuable asset to laboratory work in nuclear and atomic physics. For this reason the construction and investigation of a continuously sensitive diffusion cloud chamber has been undertaken. It is the purpose of this paper to report the design and operating characteristics of such a chamber.
- Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures
- III-V nitrides have been put to use in a variety of applications including laser diodes for modern DVD devices and for solid-state white lighting. Plasmonics has come to the foreground over the past decade as a means for increasing the internal quantum efficiency (IQE) of devices through resonant interaction with surface plasmons which exist at metal/dielectric interfaces. Increases in emission intensity of an order of magnitude have been previously reported using silver thin-films on InGaN/GaN MQWs. the dependence on resonant interaction between the plasmons and the light emitter limits the applications of plasmonics for light emission. This dissertation presents a new non-resonant mechanism based on electrostatic interaction of carriers with induced image charges in a nearby metallic nanoparticle. Enhancement similar in strength to that of plasmonics is observed, without the restrictions imposed upon resonant interactions. in this work we demonstrate several key features of this new interaction, including intensity-dependent saturation, increase in the radiative recombination lifetime, and strongly inhomogeneous light emission. We also present a model for the interaction based on the aforementioned image charge interactions. Also discussed are results of work done in the course of this research resulting in the development of a novel technique for strain measurement in light-emitting structures. This technique makes use of a spectral fitting model to extract information about electron-phonon interactions in the sample which can then be related to strain using theoretical modeling.
- A Two-Dimensional Model Study of Elastic Waves
- In seismic field operations complex problems often arise which cannot be solved mathematically. In recent years investigators in both the commercial and academic fields have begun to approach the problems of elastic wave propagation by the use of seismic scale models. This thesis discusses the results measured from simulated seismic activity on a scale model built by the researcher.
- Absolute Beta Counting Using Thick Sources
- The problem with which we shall concern ourselves in this paper is the self-scattering and self-absorption of beta particles by the source.
- Experimental Determination of the Scattering Cross-section of Ogives and Prolate Spheroids at Microwave Frequencies
- Because of the great difficulty of obtaining exact numerical values of cross-section, and because of the inherent uncertainties in interpreting and evaluating the approximate methods, accurate experimental cross-section data would be extremely useful to the radar engineer. It was with this purpose in mind that the present long-range research program in microwave scattering was undertaken. Of immediate interest were the scattering properties of the prolate spheroid, the ogive (formed by rotating the minor segment of a circle around the chord), and, for comparison, the long cylinder.
- Crystalline Polymorphism of Nitrates
- The purpose of this study was to investigate the polymorphism of a group of related compounds. Special emphasis was placed upon the temperature at which transitions occurred and a possible correlation of these temperatures with other properties of the compounds.
- Dipole Moments of Olefinic Esters
- It is the purpose of this thesis to investigate the applicability of the Debye equation to measurements dipole moments of polar compounds in dilute solutions of non-polar solvents more fully than has been done by previous workers at this institution.
- Potential Distribution of an Electrical Source-Sink Combination Along the Axis of an Infinite Cylinder
- In the present paper, an attempt is made to obtain the potential distribution in the case of two such charges, a source-sink combination, located on the axis of a bore hole drilled through an infinite, homogeneous medium.
- Scattering Cross-sections of 4/1 Prolate Spheroids
- This is a report of the second in a projected series of experiments at North Texas State College designed for obtaining information about the microwave scattering properties of various shaped objects.
- Modification of Graphene Properties: Electron Induced Reversible Hydrogenation, Oxidative Etching and Layer-by-layer Thinning
- In this dissertation, I present the mechanism of graphene hydrogenation via three different electron sources: scanning electron microscopy, e-beam irradiation and H2 and He plasma irradiation. in each case, hydrogenation occurs due to electron impact fragmentation of adsorbed water vapor from the sample preparation process. in the proposed model, secondary and backscattered electrons generated from incident electron interactions with the underlying silicon substrate are responsible for the dissociation of water vapor. Chemisorbed H species from the dissociation are responsible for converting graphene into hydrogenated graphene, graphane. These results may lead to higher quality graphane films having a larger band gap than currently reported. in addition, the dissertation presents a novel and scalable method of controllably removing single atomic planes from multi-layer graphene using electron irradiation from an intense He plasma under a positive sample bias. As the electronic properties or multi-layer graphene are highly dependent on the number of layers, n, reducing n in certain regions has many benefits. for example, a mask in conjunction with this thinning method could be used for device applications.
- Effects of Dissipation on Propagation of Surface Electromagnetic and Acoustic Waves
- With the recent emergence of the field of metamaterials, the study of subwavelength propagation of plane waves and the dissipation of their energy either in the form of Joule losses in the case of electomagnetic waves or in the form of viscous dissipation in the case of acoustic waves in different interfaced media assumes great importance. with this motivation, I have worked on problems in two different areas, viz., plasmonics and surface acoustics. the first part (chapters 2 & 3) of the dissertation deals with the emerging field of plasmonics. Researchers have come up with various designs in an efort to fabricate efficient plasmonic waveguides capable of guiding plasmonic signals. However, the inherent dissipation in the form of Joule losses limits efficient usage of surface plasmon signal. a dielectric-metal-¬dielectric planar structure is one of the most practical plasmonic structures that can serve as an efficient waveguide to guide electromagnetic waves along the metal-dielectric boundary. I present here a theoretical study of propagation of surface plasmons along a symmetric dielectric-metal-dielectric structure and show how proper orientation of the optical axis of the anisotropic substrate enhances the propagation length. an equation for propagation length is derived in a wide range of frequencies. I also show how the frequency of coupled surface plasmons can be modulated by changing the thickness of the metal film. I propose a Kronig-Penny model for the plasmonic crystal, which in the long wavelength limit, may serve as a homogeneous dielectric substrate with high anisotropy which do not exist for natural optical crystals. in the second part (chapters 4 & 5) of the dissertation, I discuss an interesting effect of extraordinary absorption of acoustic energy due to resonant excitation of Rayleigh waves in a narrow water channel clad between two metal plates. Starting from the elastic properties of the metal plates, I derive a dispersion equation that gives resonant frequencies, which coincide with those observed in the experiment that was performed by Wave Phenomena Group at Polytechnic University of Valencia, Spain. Two eigenmodes with different polarizations and phase velocities are obtained from the dispersion equation. at certain critical aperture of the channel, an interesting cutoff effect, which is unusual for an acoustic wave, is observed for one of the eigenmodes with symmetric distribution of the pressure field. the theoretical prediction of the coupling and synchronization of Rayleigh waves strongly supports the experimentally measured shift of the resonant frequencies in the transmission spectra with channel aperture. the observed high level of absorption may find applications in designing metamaterial acoustic absorbers.
- The Angular Distribution of the Deuterium-Deuterium Neutrons with 100 Kev Deuterons
- It is the purpose of this paper to present the experimental techniques used in obtaining. 3.25 MeV neutrons from the H2(d,n)He3 reaction, as well as an analysis of the experimental data.
- An Investigation for Gamma Rays Resulting from the Bombardment of As75 with 14 Mev Neutrons
- It is the purpose of this paper to set forth the method and results of studying the gamma rays resulting from the bombardment of As75 with approximately 14 Mev neutrons. The source of these neutrons was the H3(d,n)He4 reaction. The deuterons of 325 Kev energy were obtained from a Van de Graff electrostatic accelerator. A NaI scintillation spectrometer was used to determine the gamma-ray energies.
- Backscattering from Prolate Spheroids at Microwave Frequencies
- This thesis examines backscattering from prolate spheroids at microwave frequencies.
- Radar Scattering Cross-section of Triangular Corner Reflectors
- The series of experimental studies to be described has been carried out in order to determine the feasibility of using corner reflectors as laboratory standards for model cross-section measurements.
- Operation and Control of a Radiofrequency Ion Source
- This thesis examines the operation and control of a radiofrequency ion source.
- Stochastic Mechanical Systems
- To understand the phenomena associated with such stochastic processes and to predict, at least qualitatively, the behavior of mechanical systems within environments which are completely random in time, new mechanical tools are necessary. Fortunately, the derivation of these tools does not necessitate a complete departure from existing theories. In fact, they may be considered as an extension of the well-defined theory of the integral transform, in particular, the exponential Fourier integral transform.
- Design and Testing of a Coincidence System
- This paper is concerned with the design, testing and performance of a coincidence system, the proposed North Texas State College accelerator.
- D-D and D-T Neutron Excitation of Energy Levels in Cs133
- The purpose of this experiment was to make positive assignment of the Cs133 energy levels excited by the inelastic scattering of neutrons.
- Neutron Density Depression Due to an Oblate Spheroidal Detector
- In this paper, two projects have been undertaken. First, Workman's calculations have been checked to a higher degree of approximation to determine the accuracy of his method. Second, a new set of boundary conditions has been developed for obtaining solutions of the neutron diffusion equation which do not depend on the solution of the equation inside the detector.
- Quaternion Representation of Crystal Space Groups
- This investigation is designed to find quaternion operators which will generate selected space groups and which are more convenient to manipulate in some important types of problems.
- Dynamical Friction Coefficients for Plasmas Exhibiting Non-Spherical Electron Velocity Distributions
- This investigation is designed to find the net rate of decrease in the component of velocity parallel to the original direction of motion of a proton moving through an electron gas exhibiting a non-spherical velocity distribution.