Latest content added for UNT Digital Library Partner: UNT Librarieshttps://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=502017-07-12T03:17:08-05:00UNT LibrariesThis is a custom feed for browsing UNT Digital Library Partner: UNT LibrariesNonlinear Light Generation from Optical Cavities and Antennae2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984232/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984232/"><img alt="Nonlinear Light Generation from Optical Cavities and Antennae" title="Nonlinear Light Generation from Optical Cavities and Antennae" src="https://digital.library.unt.edu/ark:/67531/metadc984232/small/"/></a></p><p>Semiconductor based micro- and nano-structures grown in a systematic and controlled way using selective area growth are emerging as a promising route toward devices for integrated optical circuitry in optoelectronics and photonics field. This dissertation focuses on the experimental investigation of the nonlinear optical effects in selectively grown gallium nitride micro-pyramids that act as optical cavities, zinc oxide submicron rods and indium gallium nitride multiple quantum well core shell submicron tubes on the apex of GaN micro pyramids that act as optical antennae. Localized spatial excitation of these low dimensional semiconductor structures was optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multi-photon luminescence (MPL). The evolution of both processes are mapped along the symmetric axis of the individual structures for multiple fundamental input frequencies of light. Effects such as cavity formation of generated light, electron-hole plasma generation and coherent emission are observed. The efficiency and tunability of the frequency conversion that can be achieved in the individual structures of various geometries are estimated. By controlling the local excitation cross-section within the structures along with modulation of optical excitation intensity, the nonlinear optical process generated in these structures can be manipulated to generate coherent light in the UV-Blue region via SHG process or green emission via MPL process. The results show that these unique structures hold the potential to convert red input pulsed light into blue output pulsed light which is highly directional.</p>Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955084/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955084/"><img alt="Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates" title="Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates" src="https://digital.library.unt.edu/ark:/67531/metadc955084/small/"/></a></p><p>In this work, we study the use of a spatial light modulator (SLM) for local manipulation of phase in interfering laser beams to fabricate photonic crystal templates with embedded, engineered defects. A SLM displaying geometric phase patterns was used as a digitally programmable phase mask to fabricate 4-fold and 6-fold symmetric photonic crystal templates. Through pixel-by-pixel phase engineering, digital control of the phases of one or more of the interfering beams was demonstrated, thus allowing change in the interference pattern. The phases of the generated beams were programmed at specific locations, resulting in defect structures in the fabricated photonic lattices such as missing lattice line defects, and single-motif lattice defects in dual-motif lattice background. The diffraction efficiency from the phase pattern was used to locally modify the filling fraction in holographically fabricated structures, resulting in defects with a different fill fraction than the bulk lattice. Through two steps of phase engineering, a spatially variant lattice defect with a 90° bend in a periodic bulk lattice was fabricated. Finally, by reducing the relative phase shift of the defect line and utilizing the different diffraction efficiency between the defect line and the background phase pattern, desired and functional defect lattices can be registered into the background lattice through direct imaging of the designed phase patterns.</p>Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955114/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955114/"><img alt="Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes" title="Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes" src="https://digital.library.unt.edu/ark:/67531/metadc955114/small/"/></a></p><p>We investigate the conditions under which electron irradiation of single-walled carbon nanotube (SWCNT) bundles with 2 keV electrons produces an increase in the Raman D peak. We find that an increase in the D peak does not occur when SWCNTs are preheated in situ at 600 C for 1 h in ultrahigh vacuum (UHV) before irradiation is performed. Exposing SWCNTs to air or other gases after preheating in UHV and before irradiation results in an increase in the D peak. Small diameter SWCNTs that are not preheated or preheated and exposed to air show a significant increase in the D and G bands after irradiation. X-ray photoelectron spectroscopy shows no chemical shifts in the C1s peak of SWCNTs that have been irradiated versus SWCNTs that have not been irradiated, suggesting that the increase in the D peak is not due to chemisorption of adsorbates on the nanotubes.</p>Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955104/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955104/"><img alt="Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films" title="Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films" src="https://digital.library.unt.edu/ark:/67531/metadc955104/small/"/></a></p><p>Among the well-known methods to form or modify the composition and physical properties of thin films, ion implantation has shown to be a very powerful technique. In particular, ion beam syntheses of binary iron silicide have been studied by several groups. Further, the interests in transition metal silicide systems are triggered by their potential use in advanced silicon based opto-electronic devices. In addition, ternary silicides have been by far less studied than their binary counterparts despite the fact that they have interesting magnetic and electronic properties. In this study, we investigate ion beam synthesis of Fe-Si binary structures and Fe-Co-Si ternary structures. This work involves fundamental investigation into development of a scalable synthesis process involving binary and ternary transitional metal silicide thin films and Nano-structures using low energy ion beams.
Binary structures were synthesized by implanting Fe- at 50 keV energy. Since ion implantation is a dynamic process, Dynamic simulation techniques were used in these studies to determine saturation fluences for ion implantation. Also, static and dynamic simulation results were compared with experimental results. The outcome of simulations and experimental results indicate, dynamic simulation codes are more suitable than static version of the TRIM to simulate high fluence, low energy and, heavy ion implantation processes. Furthermore, binary Fe-Si phase distribution was determined at different implantation fluences and annealing temperatures. A higher fluence implantation at 2.16×1017 atoms/cm2 and annealing at 500 oC showed three different Fe-Si phase formations (β-FeSi2, FeSi and Fe3Si) in substrate. Further, annealing the samples at 800 oC for 60 minutes converted the Fe3Si phase into FeSi2 and FeSi phases. As an extension, a second set of Fe- ion implantations was carried with the same parameters while the substrate was placed under an external magnetic field. External magnetic fields stimulate the formation of magnetic phase centers in the substrate. X-ray diffraction (XRD) results shows formation of ferromagnetic Fe3Si phase in the Si matrix after annealing at 500 oC for 60 minutes. In addition, X-ray photoelectron spectra (XPS) provide further evidence for ferromagnetic metallic behavior of Fe3Si in the substrate. Ternary Fe-Co-Si structures were synthesized by implanting Fe- & Co- into a Si (100) substrate at an energy of 50 keV at saturation fluences. Both Fe- & Co- co-implantation were performed under external magnetic fields to enhance magnetic phase formation. Fe(1-x)CoxSi B20-type cubic structure can be synthesized on Si(100) substrate with 0.4≤x≤0.55 concentration range using ion implantation under external magnetic field. Moreover, magnetic measurement indicates a possible magnetic phase transformation at ~50 K. Further, XPS results also provide evidence for metallic & ferromagnetic properties in the thin film structure</p>Ultrasensitive Technique for Measurement of Two-Photon Absorption2016-11-15T10:54:02-06:00https://digital.library.unt.edu/ark:/67531/metadc935773/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc935773/"><img alt="Ultrasensitive Technique for Measurement of Two-Photon Absorption" title="Ultrasensitive Technique for Measurement of Two-Photon Absorption" src="https://digital.library.unt.edu/ark:/67531/metadc935773/small/"/></a></p><p>Intensive demands have arisen to characterize nonlinear optical properties of materials for applications involving optical limiters, waveguide switches and bistable light switches. The technique of Pulse Delay Modulation is described which can monitor nonlinear changes in transmission with shot noise limited signal-to-noise ratios even in the presence of large background signals. The theoretical foundations of the experiment are presented followed by actual measurements of beam depletion due to second harmonic generation in a LiIO3 crystal and two-photon absorption in the semiconductor ZnSe. Sensitivity to polarization rotation arising from the Kerr Effect in carbon disulfide, saturable absorber relaxation in modelocking dyes and photorefractive effects in ZnSe are demonstrated. The sensitivity of Pulse Delay Modulation is combined with Fabry-Perot enhancement to allow the measurement of two-photon absorption in a 0.46pm thick interference filter spacer layer. Also included is a study of nonlinear optical limiting arising from dielectric breakdown in gases.</p>Investigation of the Effects of Compressive Uniaxial Stress on the Hole Carriers in P-type InSb2016-11-15T10:54:02-06:00https://digital.library.unt.edu/ark:/67531/metadc935820/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc935820/"><img alt="Investigation of the Effects of Compressive Uniaxial Stress on the Hole Carriers in P-type InSb" title="Investigation of the Effects of Compressive Uniaxial Stress on the Hole Carriers in P-type InSb" src="https://digital.library.unt.edu/ark:/67531/metadc935820/small/"/></a></p><p>The influence of uniaxial compression upon the Hall effect ad resistivity of cadmium-doped samples of InSb at 77 K, 64 K, and 12 K are reported. Unilaxial compressions as high as 6 kbar were applied to samples oriented in the {001} and {110} directions. The net hole concentration of the samples were about 5x10^13 cm^-3 at 77 K as determined from the Hall coefficient at 24 kilogauss. The net concentration of hole carriers decreases and then increases exponentially with stress at 77 k and 64 k, while at 12 k there is only a monotonic increase of carrier concentration with stress. Analysis of the hole concentration as a function of stress shows the presence of a deep acceptor level located about 90 meV above the valence band edge in additionb to the 10 meV vadmium acceptor level. The shallow acceptor level does not split with stress. The hole density data is represented very well by models which describe both the variation in the net density of states and motion of the acceptor levels as a function of stress.</p>Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849779/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849779/"><img alt="Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production" title="Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production" src="https://digital.library.unt.edu/ark:/67531/metadc849779/small/"/></a></p><p>Comparisons between measurements of the ground-state hyperfine structure and gravitational acceleration of hydrogen and antihydrogen could provide a test of fundamental physical theories such as CPT (charge conjugation, parity, time-reversal) and gravitational symmetries. Currently, antihydrogen traps are based on Malmberg-Penning traps. The number of antiprotons in Malmberg-Penning traps with sufficiently low energy to be suitable for trappable antihydrogen production may be reduced by the electrostatic space charge of the positrons and/or collisions among antiprotons. Alternative trap designs may be needed for future antihydrogen experiments. A computational tool is developed to simulate charged particle motion in customizable magnetic fields generated by combinations of current loops and current lines. The tool is used to examine charged particle confinement in two systems consisting of dual, levitated current loops. The loops are coaxial and arranged to produce a magnetic null curve. Conditions leading to confinement in the system are quantified and confinement modes near the null curve and encircling one or both loops are identified. Furthermore, the tool is used to examine and quantify charged particle motion parallel to the null curve in the large radius limit of the dual, levitated current loops. An alternative to new trap designs is to identify the effects of the positron space in existing traps and to find modes of operation where the space charge is beneficial. Techniques are developed to apply the Boltzmann density relation along curved magnetic field lines. Equilibrium electrostatic potential profiles for a positron plasma are computed by solving Poisson's equation using a finite-difference method. Equilibria are computed in a model Penning trap with an axially varying magnetic field. Also, equilibria are computed for a positron plasma in a model of the ALPHA trap. Electric potential wells are found to form self-consistently. The technique is expanded to compute equilibria for a two-species plasma with an antiproton plasma confined by the positron space charge. The two-species equilibria are used to estimate timescales associated with three-body recombination, losses due to collisions between antiprotons, and temperature equilibration. An equilibrium where the three-body recombination rate is the smallest is identified.</p>A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849601/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849601/"><img alt="A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation" title="A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation" src="https://digital.library.unt.edu/ark:/67531/metadc849601/small/"/></a></p><p>In this work over 200 temperature proxy data sets have been analyzed to determine if periodic and or quasi-periodic patterns exist in the data sets. References to the journal articles where data are recorded are provided. Chapter 1 serves an introduction to the problem of temperature determination in providing information on how various proxy data sources are derived. Examples are given of the techniques followed in producing proxy data that predict temperature for each method used. In chapter 2 temperature proxy data spanning the last 4000 years, from 2,000 BCE to 2,000 CE, are analyzed to determine if overarching patterns exist in proxy data sets. An average of over 100 proxy data sets was used to produce Figure 4. An overview of the data shows that several “peaks” can be identified. The data were then subjected to analysis using a series of frequency modulated cosine waves. This analysis led to a function that can be expressed by equation 3. The literature was examined to determine what mathematical models had been published to fit the experimental proxy data for temperature. A number of attempts have been made to fit data from limited data sets with some degree of success. Some other papers have used a sinusoidal function to best fit the changes in the temperature. After consideration of many published papers and reviewing long time streams of proxy data that appeared to have sine wave patterns, a new model was proposed for trial. As the patterns observed showed “almost” repeating sine cycles, a frequency modulated sine wave was chosen to obtain a best fit function. Although other papers have used a sinusoidal function to best fit the changes in the temperature, the “best fit” was limited. Thus, it was decided that a frequency modulated sine wave may be a better model that would provide a more precise fit. This proved to be the case and the more than 240 temperature proxy data sets were analyzed using Equation 3. In chapter 3 the time span for the proxy data was extended to cover the period of time 12,000 BCE to 2000 CE. The data were then tested by using the equation above to search for periodic/quasi-periodic patterns. These results are summarized under select conditions of time periods. In chapter 4 the interval of time is extended over 1,000,000 years of time to test for long period “periodic” changes in global temperature. These results are provided for overall analysis. The function f(x) as described above was used to test for periodic/quasi-periodic changes in the data. Chapter 5 provides an analysis of temperature proxy data for an interval of time of 3,000,000 years to establish how global temperature has varied during the last three million years. Some long-term quasi-periodic patterns are identified. Chapter 6 provides a summation of the model proposed for global temperature that can be expected if similar trends continue over future years as have prevailed for the past few million years. Data sets that were used in this work are tabulated in the appendices of this paper.</p>Quantum Coherent Control and Propagation in Lambda System2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849750/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849750/"><img alt="Quantum Coherent Control and Propagation in Lambda System" title="Quantum Coherent Control and Propagation in Lambda System" src="https://digital.library.unt.edu/ark:/67531/metadc849750/small/"/></a></p><p>Strong coherence in quasi-resonant laser driven system interferes with effective relaxations, resulting in behaviors like, coherent population trapping and Electromagnetically induced transparency. The Raman system can optimize this utilizing excited coherence in the lambda system when exposed to counter- intuitive pump-stokes pulses. The phenomenon can result in complete population transfer between vibrational levels called Stimulated Raman adiabatic passage(STIRAP). STIRAP and CHIRAP have been studied with Gaussian and chirped pulses. The optical propagation effects in dense medium for these phenomenon is studied to calculate the limitations and induced coherences. Further, the effect of rotational levels has been investigated. The molecular vibrational coherence strongly depends on the effect of rotational levels. The change in coherence interaction for ro-vibrational levels are reported and explained. We have considered the effects on the phase of radiation related to rotational mechanical motion of quantum system by taking advantages in ultra strong dispersion medium provided by quantum coherence in lambda system. The enhanced Fizeau effect on a single atom is observed.</p>Fractional Calculus and Dynamic Approach to Complexity2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822832/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822832/"><img alt="Fractional Calculus and Dynamic Approach to Complexity" title="Fractional Calculus and Dynamic Approach to Complexity" src="https://digital.library.unt.edu/ark:/67531/metadc822832/small/"/></a></p><p>Fractional calculus enables the possibility of using real number powers or complex number powers of the differentiation operator. The fundamental connection between fractional calculus and subordination processes is explored and affords a physical interpretation for a fractional trajectory, that being an average over an ensemble of stochastic trajectories. With an ensemble average perspective, the explanation of the behavior of fractional chaotic systems changes dramatically. Before now what has been interpreted as intrinsic friction is actually a form of non-Markovian dissipation that automatically arises from adopting the fractional calculus, is shown to be a manifestation of decorrelations between trajectories. Nonlinear Langevin equation describes the mean field of a finite size complex network at criticality. Critical phenomena and temporal complexity are two very important issues of modern nonlinear dynamics and the link between them found by the author can significantly improve the understanding behavior of dynamical systems at criticality. The subject of temporal complexity addresses the challenging and especially helpful in addressing fundamental physical science issues beyond the limits of reductionism.</p>Nonlinear and Quantum Optics Near Nanoparticles2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822820/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822820/"><img alt="Nonlinear and Quantum Optics Near Nanoparticles" title="Nonlinear and Quantum Optics Near Nanoparticles" src="https://digital.library.unt.edu/ark:/67531/metadc822820/small/"/></a></p><p>We study the behavior of electric fields in and around dielectric and metal nanoparticles, and prepare the ground for their applications to a variety of systems viz. photovoltaics, imaging and detection techniques, and molecular spectroscopy. We exploit the property of nanoparticles being able to focus the radiation field into small regions and study some of the interesting nonlinear, and quantum coherence and interference phenomena near them. The traditional approach to study the nonlinear light-matter interactions involves the use of the slowly varying amplitude approximation (SVAA) as it simplifies the theoretical analysis. However, SVVA cannot be used for systems which are of the order of the wavelength of the light. We use the exact solutions of the Maxwell's equations to obtain the fields created due to metal and dielectric nanoparticles, and study nonlinear and quantum optical phenomena near these nanoparticles. We begin with the theoretical description of the electromagnetic fields created due to the nonlinear wavemixing process, namely, second-order nonlinearity in an nonlinear sphere. The phase-matching condition has been revisited in such particles and we found that it is not satisfied in the sphere. We have suggested a way to obtain optimal conditions for any type and size of material medium. We have also studied the modifications of the electromagnetic fields in a collection of nanoparticles due to strong near field nonlinear interactions using the generalized Mie theory for the case of many particles applicable in photovoltaics (PV). We also consider quantum coherence phenomena such as modification of dark states, stimulated Raman adiabatic passage (STIRAP), optical pumping in $4$-level atoms near nanoparticles by using rotating wave approximation to describe the Hamiltonian of the atomic system. We also considered the behavior of atomic and the averaged atomic polarization in $7$-level atoms near nanoparticles. This could be used as a prototype to study any $n-$level atomic system experimentally in the presence of ensembles of quantum emitters. In the last chapter, we suggested a variant of a pulse-shaping technique applicable in stimulated Raman spectroscopy (SRS) for detection of atoms and molecules in multiscattering media. We used discrete-dipole approximation to obtain the fields created by the nanoparticles.</p>Variational Calculations of Positronium Scattering with Hydrogen2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822803/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822803/"><img alt="Variational Calculations of Positronium Scattering with Hydrogen" title="Variational Calculations of Positronium Scattering with Hydrogen" src="https://digital.library.unt.edu/ark:/67531/metadc822803/small/"/></a></p><p>Positronium-hydrogen (Ps-H) scattering is of interest, as it is a fundamental four-body Coulomb problem. We have investigated low-energy Ps-H scattering below the Ps(n=2) excitation threshold using the Kohn variational method and variants of the method with a trial wavefunction that includes highly correlated Hylleraas-type short-range terms. We give an elegant formalism that combines all Kohn-type variational methods into a single form. Along with this, we have also developed a general formalism for Kohn-type matrix elements that allows us to evaluate arbitrary partial waves with a single codebase. Computational strategies we have developed and use in this work will also be discussed.With these methods, we have computed phase shifts for the first six partial waves for both the singlet and triplet states. The 1S and 1P phase shifts are highly accurate results and could potentially be viewed as benchmark results. Resonance positions and widths for the 1S-, 1P-, 1D-, and 1F-waves have been calculated.We present elastic integrated, elastic differential, and momentum transfer cross sections using all six partial waves and note interesting features of each. We use multiple effective range theories, including several that explicitly take into account the long-range van der Waals interaction, to investigate scattering lengths for the 1,3S and 1,3P partial waves and effective ranges for the 1,3S-wave.</p>A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium2016-03-04T16:14:01-06:00https://digital.library.unt.edu/ark:/67531/metadc804828/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc804828/"><img alt="A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium" title="A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium" src="https://digital.library.unt.edu/ark:/67531/metadc804828/small/"/></a></p><p>We perform high precision measurements of an isotope shift between the two stable isotopes of helium. We use laser excitation of the 2^3 S_1-2^3 P_0 transition at 1083 nm in a metastable beam of 3He and 4He atoms. A newly developed tunable laser frequency selector along with our previous electro-optic frequency modulation technique provides extremely reliable, adaptable, and precise frequency and intensity control. The intensity control contributes negligibly to overall experimental uncertainty by stabilizing the intensity of the required sideband and eliminating the unwanted frequencies generated during the modulation of 1083 nm laser carrier frequency. The selection technique uses a MEMS based fiber switch and several temperature stabilized narrow band (~3 GHz) fiber gratings. A fiber based optical circulator and an inline fiber amplifier provide the desired isolation and the net gain for the selected frequency. Also rapid (~2 sec.) alternating measurements of the 2^3 S_1-2^3 P_0 interval for both species of helium is achieved with a custom fiber laser for simultaneous optical pumping. A servo-controlled retro-reflected laser beam eliminates residual Doppler effects during the isotope shift measurement. An improved detection design and software control makes negligible subtle potential biases in the data collection. With these advances, combined with new internal and external consistency checks, we are able to obtain results consistent with the best previous measurements, but with substantially improved precision. Our measurement of the 2^3 S_1-2^3 P_0 isotope shift between 3He and 4He is 31 097 535.2 (5) kHz. The most recent theoretic calculation combined with this measurement yields a new determination for nuclear size differences between 3He and 4He: ∆r_c=0.292 6 (1)_exp (8)_th (52)_exp fm, with a precision of less than a part in 〖10〗^4 coming from the experimental uncertainty (first parenthesis), and a part in 〖10〗^3 coming from theory. This value is consistent with electron scattering measurement, but a factor of 10 more precise. It is inconsistent (4 sigma) with a recent isotope shift measurement on another helium transition (2^1 S_0-2^3 S_1). Comparisons with ongoing muonic helium measurements may provide clues to the origin of what is currently called the proton puzzle: electronic and muonic measurements of the proton size do not agree. In the future, the experimental improvements described here can be used for higher precision tests of atomic theory and quantum electrodynamics, as well as an important atomic physics source of the fine structure constant.</p>Complex Numbers in Quantum Theory2016-03-04T16:14:01-06:00https://digital.library.unt.edu/ark:/67531/metadc804988/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc804988/"><img alt="Complex Numbers in Quantum Theory" title="Complex Numbers in Quantum Theory" src="https://digital.library.unt.edu/ark:/67531/metadc804988/small/"/></a></p><p>In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.</p>Electromagnetically Modulated Sonic Structures2016-02-02T13:35:12-06:00https://digital.library.unt.edu/ark:/67531/metadc799496/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc799496/"><img alt="Electromagnetically Modulated Sonic Structures" title="Electromagnetically Modulated Sonic Structures" src="https://digital.library.unt.edu/ark:/67531/metadc799496/small/"/></a></p><p>Phononic crystals are structures composed of periodically arranged scatterers in a background medium that affect the transmission of elastic waves. They have garnered much interest in recent years for their macro-scale properties that can be modulated by the micro-scale components. The elastic properties of the composite materials, the contrast in the elastic properties of the composite materials, and the material arrangement all directly affect how an elastic wave will behave as it propagates through the sonic structure. The behavior of an elastic wave in a periodic structure is revealed in its transmission bandstructure, and modification of any the elastic parameters will result in tuning of the band structure. In this dissertation, a phononic crystal with properties that can be modulated using electromagnetic radiation, and more specifically, radio-frequency (RF) light will be presented.</p>Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals2016-02-02T13:35:12-06:00https://digital.library.unt.edu/ark:/67531/metadc799538/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc799538/"><img alt="Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals" title="Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals" src="https://digital.library.unt.edu/ark:/67531/metadc799538/small/"/></a></p><p>Performance and reliability of solid state laser diodes in the IR region exceeds those in the visible and UV part of the light spectrum. Single frequency visible and UV laser diodes with higher than 500 mW power are not available commercially. However we successfully stabilized a multi-longitudinal mode IR laser to 860 mW single frequency. This means high efficiency harmonic generation using this laser can produce visible and UV laser light not available otherwise. In this study we examined three major leading nonlinear crystals: PPMgO:SLN, PPKTP and PPMgO:SLT to generate blue light by second harmonic generation. We achieved record high net conversion efficiencies 81.3% using PPMgO:SLT (~500 mW out), and 81.1% using PPKTP (~700 mW out). In both these cases an external resonance buildup cavity was used. We also studied a less complicated single pass waveguide configuration (guided waist size of ~ 5 um compared to ~60 um) to generate blue. With PPMgO:SLN we obtained net 40.4% and using PPKT net 6.8% (110mW and 10.1 mW respectively).</p>Interaction of Plasmons and Excitons for Low-Dimension Semiconductors2016-02-02T13:35:12-06:00https://digital.library.unt.edu/ark:/67531/metadc799475/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc799475/"><img alt="Interaction of Plasmons and Excitons for Low-Dimension Semiconductors" title="Interaction of Plasmons and Excitons for Low-Dimension Semiconductors" src="https://digital.library.unt.edu/ark:/67531/metadc799475/small/"/></a></p><p>The effects of surface plasmon for InGaN/GaN multi-quantum wells and ZnO nanoparticles optical linear and nonlinear emission efficiency had been experimentally studied. Due to the critical design for InGaN MQWs with inverted hexagonal pits based on GaN, both contribution of surface plasmon effect and image charge effect at resonant and off resonant frequencies were experimentally and theoretically investigated. With off- resonant condition, the InGaN MQWs emission significantly enhanced by metal nanoparticles. This enhancement was caused by the image charge effect, due to the accumulation of carriers to NPs region. When InGaN emission resonated with metal particles SP modes, surface Plasmon effect dominated the emission process. We also studied the surface plasmon effect for ZnO nanoparticles nonlinear optical processes, SHG and TPE. Defect level emission had more contribution at high incident intensity. Emissions are different for pumping deep into the bulk and near surface. A new assumption to increase the TPE efficiency was studied. We thought by using Au nanorods localized surface plasmon mode to couple the ZnO virtual state, the virtual state’s life time would be longer and experimentally lead the emission enhancement. We studied the TPE phenomena at high and near band gap energy. Both emission intensity and decay time results support our assumption. Theoretically, the carriers dynamic mechanism need further studies.</p>Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency2016-02-02T13:35:12-06:00https://digital.library.unt.edu/ark:/67531/metadc799502/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc799502/"><img alt="Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency" title="Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency" src="https://digital.library.unt.edu/ark:/67531/metadc799502/small/"/></a></p><p>In recent years a great deal of interest has been focused on the synthesis of transitional metal (e.g. Ag, Cu, Fe, Au) nanosystems at the surface to sub-surface regions of Si and SiO2 matrices for fundamental understanding of their structures as well as for development of technological applications with enhanced electronic and optical properties. The applications of the metal nanoparticle or nanocluster (NC) systems range from plasmonics, photovoltaic devices, medical, and biosensors. In all of these applications; the size, shape and distribution of the metallic NCs in the silicon matrix play a key role. Low energy ion implantation followed by thermal annealing (in vacuum or gas environment) is one of the most suitable methods for synthesis of NCs at near surfaces to buried layers below the surfaces of the substrates. This technique can provide control over depth and concentration of the implanted ions in the host matrix. The implanted low energy metal ions initially amorphizes the Si substrates while being distributed at a shallow depth near the substrate surface. When subject to thermal annealing, the implanted ions agglomerate to form clusters of different sizes at different depths depending upon the fluence. However, for the heavier ions implanted with high fluences (~1×1016 - 1×1017 atoms/cm2), there lies challenges for accurately predicting the distribution of the implanted ions due to sputtering of the surface as well as redistribution of the implants within the host matrix. In this dissertation, we report the investigation of the saturation of the concentration of the implanted ion species in the depth profiles with low energies (< 80 keV) metal ions (Ag and Au) in Si (100), while studying the dynamic changes during the ion implantation. Multiple low energies (30-80 keV) Ag ions with different fluences were sequentially implanted into commercially available Si wafers in order to facilitate the formation of Ag NCs with a wide ion distributions range. The light absorption profile according to different sizes of NCs at the near-surface layers in Si were investigated. We have investigated the formation of Ag NCs in the Si matrix as a function of implantation and thermal annealing parameters. The absorbance of light is increased in Ag implanted Si with a significant increase in the current collection in I-V (current-voltage) photo switching measurements. The experimental photovoltaic cells fabricated with the Ag implanted Si samples were optically characterized under AM (air mass) 1.5 solar radiation conditions (~1.0 kW/m2). An enhancement in the charge collection were measured in the annealed samples, where prominent Ag NCs were formed in the Si matrix compared to the as-implanted samples with the amorphous layer. The characterization techniques such as Rutherford Backscattering Spectroscopy, XPS-depth profiling, transmission electron microscopy, optical absorption, and I-V (current-voltage) photo switching measurements were employed to understand the underlying science in the observed properties. The results of these investigations are discussed in this research.</p>Electrolysis of Aluminum Solutions in a Magnetic Field2016-01-14T20:34:50-06:00https://digital.library.unt.edu/ark:/67531/metadc798392/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc798392/"><img alt="Electrolysis of Aluminum Solutions in a Magnetic Field" title="Electrolysis of Aluminum Solutions in a Magnetic Field" src="https://digital.library.unt.edu/ark:/67531/metadc798392/small/"/></a></p><p>This investigation is an attempt to verify the original work done by George Antonoff and Anne Rowley, and to contribute specific data on the action of a magnetic field on aluminum cells. Experiments of the type they have described have been performed and an extensive set of data has been collected. It was thought that if the results of Antonoff and Rowley could be duplicated, further investigation would be warranted. However, the experiments have produced negative results. These results are described in detail in these chapters.</p>Enhancements of Mechanical, Thermal Stability, and Tribological Properties by Addition of Functionalized Reduced Graphene Oxide in Epoxy2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc699889/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc699889/"><img alt="Enhancements of Mechanical, Thermal Stability, and Tribological Properties by Addition of Functionalized Reduced Graphene Oxide in Epoxy" title="Enhancements of Mechanical, Thermal Stability, and Tribological Properties by Addition of Functionalized Reduced Graphene Oxide in Epoxy" src="https://digital.library.unt.edu/ark:/67531/metadc699889/small/"/></a></p><p>The effects of octadecylamine-functionalized reduced graphene oxide (FRGO) on the frictional and wear properties of diglycidylether of bisphenol-A (DGEBA) epoxy are studied using a pin-on-disk tribometer. It was observed that the addition of FRGO significantly improves the tribological, mechanical, and thermal properties of epoxy matrix. Graphene oxide (GO) was functionalized with octadecylamine (ODA), and then reduction of oxygen-containing functional groups was carried out using hydrazine monohydrate. The Raman and x-ray photoelectron spectroscopy studies confirm significant reduction in oxygen-containing functional groups and formation of ODA functionalized reduced GO. The nanocomposites are prepared by adding 0.1, 0.2, 0.5 and 1.0 wt % of FRGO to the epoxy. The addition of FRGO increases by more than an order of magnitude the sliding distance during which the dynamic friction is ≤ 0.1. After this distance, the friction sharply increases to the range of 0.4 - 0.5. We explain the increase in sliding distance during which the friction is low by formation of a transfer film from the nanocomposite to the counterface. The wear rates in the low and high friction regimes are approximately 1.5 x 10-4 mm3/N·m and 5.5 x 10-4 mm3/N·m, respectively. The nanocomposites exhibit a 74 % increase in Young’s modulus with 0.5 wt. % of FRGO, and an increase in glass transition and thermal degradation temperatures.</p>Analysis of Biological Materials Using a Nuclear Microprobe2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc700099/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc700099/"><img alt="Analysis of Biological Materials Using a Nuclear Microprobe" title="Analysis of Biological Materials Using a Nuclear Microprobe" src="https://digital.library.unt.edu/ark:/67531/metadc700099/small/"/></a></p><p>The use of nuclear microprobe techniques including: Particle induced x-ray emission (PIXE) and Rutherford backscattering spectrometry (RBS) for elemental analysis and quantitative elemental imaging of biological samples is especially useful in biological and biomedical research because of its high sensitivity for physiologically important trace elements or toxic heavy metals. The nuclear microprobe of the Ion Beam Modification and Analysis Laboratory (IBMAL) has been used to study the enhancement in metal uptake of two different plants. The roots of corn (Zea mays) have been analyzed to study the enhancement of iron uptake by adding Fe (II) or Fe (III) of different concentrations to the germinating medium of the seeds. The Fe uptake enhancement effect produced by lacing the germinating medium with carbon nanotubes has also been investigated. The aim of this investigation is to ensure not only high crop yield but also Fe-rich food products especially from calcareous soil which covers 30% of world’s agricultural land. The result will help reduce iron deficiency anemia, which has been identified as the leading nutritional disorder especially in developing countries by the World Health Organization. For the second plant, Mexican marigold (Tagetes erecta), the effect of an arbuscular mycorrhizal fungi (Glomus intraradices) for the improvement of lead-phytoremediation of lead contaminated soil has been investigated. Phytoremediation provides an environmentally safe technique of removing toxic heavy metals (like lead), which can find their way into human food, from lands contaminated by human activities like mining or by natural disasters like earthquakes. The roots of Mexican marigold have been analyzed to study the role of arbuscular mycorrhizal fungi in enhancement of lead uptake from the contaminated rhizosphere.</p>Electrical Conduction Mechanisms in the Disordered Material System P-type Hydrogenated Amorphous Silicon2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc700106/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc700106/"><img alt="Electrical Conduction Mechanisms in the Disordered Material System P-type Hydrogenated Amorphous Silicon" title="Electrical Conduction Mechanisms in the Disordered Material System P-type Hydrogenated Amorphous Silicon" src="https://digital.library.unt.edu/ark:/67531/metadc700106/small/"/></a></p><p>The electrical and optical properties of boron doped hydrogenated amorphous silicon thin films (a-Si) were investigated to determine the effect of boron and hydrogen incorporation on carrier transport. The a-Si thin films were grown by plasma enhanced chemical vapor deposition (PECVD) at various boron concentrations, hydrogen dilutions, and at differing growth temperatures. The temperature dependent conductivity generally follows the hopping conduction model. Above a critical temperature, the dominant conduction mechanism is Mott variable range hopping conductivity (M-VRH), where p = ¼, and the carrier hopping depends on energy. However, at lower temperatures, the coulomb interaction between charge carriers becomes important and Efros-Shklosvkii variable hopping (ES-VRH) conduction, where p=1/2, must be included to describe the total conductivity. To correlate changes in electrical conductivity to changes in the local crystalline order, the transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of growth temperature, boron, and hydrogen incorporation. With an increase of hydrogen and/or growth temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of short range order. It is seen that there is a direct correlation between the electrical conductivity and changes in the short and mid-range order resulting from the passivation of defects by hydrogen and the creation of trap states by boron. This work was done under the ARO grant W911NF-10-1-0410, William W. Clark Program Manager. The samples were provided by L-3 Communications.</p>Temporal Complexity and Stochastic Central Limit Theorem2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc700093/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc700093/"><img alt="Temporal Complexity and Stochastic Central Limit Theorem" title="Temporal Complexity and Stochastic Central Limit Theorem" src="https://digital.library.unt.edu/ark:/67531/metadc700093/small/"/></a></p><p>Complex processes whose evolution in time rests on the occurrence of a large and random number of intermittent events are the systems under study. The mean time distance between two consecutive events is infinite, thereby violating the ergodic condition and activating at the same time a stochastic central limit theorem that explains why the Mittag-Leffler function is a universal property of nature. The time evolution of these complex systems is properly generated by means of fractional differential equations, thus leading to the interpretation of fractional trajectories as the average over many random trajectories, each of which fits the stochastic central limit theorem and the condition for the Mittag-Leffler universality. Additionally, the effect of noise on the generation of the Mittag-Leffler function is discussed. Fluctuations of relatively weak intensity can conceal the asymptotic inverse power law behavior of the Mittag-Leffler function, providing a reason why stretched exponentials are frequently found in nature. These results afford a more unified picture of complexity resting on the Mittag-Leffler function and encompassing the standard inverse power law definition.</p>Studies of Charged Particle Dynamics for Antihydrogen Synthesis2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc699934/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc699934/"><img alt="Studies of Charged Particle Dynamics for Antihydrogen Synthesis" title="Studies of Charged Particle Dynamics for Antihydrogen Synthesis" src="https://digital.library.unt.edu/ark:/67531/metadc699934/small/"/></a></p><p>Synthesis and capture of antihydrogen in controlled laboratory conditions will enable precise studies of neutral antimatter. The work presented deals with some of the physics pertinent to manipulating charged antiparticles in order to create neutral antimatter, and may be applicable to other scenarios of plasma confinement and charged particle interaction. The topics covered include the electrostatic confinement of a reflecting ion beam and the transverse confinement of an ion beam in a purely electrostatic configuration; the charge sign effect on the Coulomb logarithm for a two component (e.g., antihydrogen) plasma in a Penning trap as well as the collisional scattering for binary Coulomb interactions that are cut off at a distance different than the Debye length; and the formation of magnetobound positronium and protonium.</p>Sputtering of Bi and Preferential Sputtering of an Inhomogeneous Alloy2015-08-21T05:42:39-05:00https://digital.library.unt.edu/ark:/67531/metadc700021/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc700021/"><img alt="Sputtering of Bi and Preferential Sputtering of an Inhomogeneous Alloy" title="Sputtering of Bi and Preferential Sputtering of an Inhomogeneous Alloy" src="https://digital.library.unt.edu/ark:/67531/metadc700021/small/"/></a></p><p>Angular distributions and total yields of atoms sputtered from bismuth targets by normally incident 10 keV -50 keV Ne+ and Ar+ ions have been measured both experimentally and by computer simulation. Polycrystalline Bi targets were used for experimental measurements. The sputtered atoms were collected on high purity aluminum foils under ultra-high vacuum conditions, and were subsequently analyzed using Rutherford backscattering spectroscopy. The Monte-Carlo based SRIM code was employed to simulate angular distributions of sputtered Bi atoms and total sputtering yields of Bi to compare with experiment. The measured sputtering yields were found to increase with increasing projectile energy for normally incident 10 keV - 50 keV Ne+ and Ar+ ions. The shapes of the angular distributions of sputtered Bi atoms demonstrated good agreement between experiment and simulation in the present study. The measured and simulated angular distributions of sputtered Bi exhibited an over-cosine tendency. The measured value of the degree of this over-cosine nature was observed to increase with increasing incident Ne+ ion energy, but was not strongly dependent on incident Ar+ ion energy. The differential angular sputtering yield and partial sputtering yields due to Ar ion bombardment of an inhomogeneous liquid Bi:Ga alloy have been investigated, both experimentally and by computer simulation. Normally incident 25 keV and 50 keV beams of Ar+ were used to sputter a target of 99.8 at% Ga and 0.2 at% Bi held at 40° C in ultra-high vacuum (UHV), under which conditions the alloy is known to exhibit extreme Gibbsian surface segregation that produces essentially a monolayer of Bi atop the bulk liquid. Angular distributions of sputtered neutrals and partial sputtering yields obtained from the conversion of areal densities of Bi and Ga atoms on collector foils were determined. The Monte-Carlo based SRIM code was employed to simulate the experiment and obtain the angular distribution of sputtered components. The angular distribution of sputtered Ga atoms, originating from underneath the surface monolayer, was measured to be sharply peaked in angle about the surface normal direction compared to the Bi atoms originating from surface monolayer. The simulation study produced contradicting results, where the species originating from surface monolayer was strongly peaked around the surface normal compared to the species originating from beneath the surface monolayer.</p>Microwave Line Widths of the Asymmetric Top Formic Acid Molecule2015-06-24T09:39:17-05:00https://digital.library.unt.edu/ark:/67531/metadc663604/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc663604/"><img alt="Microwave Line Widths of the Asymmetric Top Formic Acid Molecule" title="Microwave Line Widths of the Asymmetric Top Formic Acid Molecule" src="https://digital.library.unt.edu/ark:/67531/metadc663604/small/"/></a></p><p>This work consisted of an experimental investigation of the formic acid (HCOOH) molecule's rotational spectrum. Measurements of line widths were obtained for J = 5, 12, 13, 19, and 20 for a pressure range from 1 to 10 microns. A linear behavior between Av and p was observed as predicted by theory. The line width parameter Avp was observed to depend on the quantum number J. Hard sphere collision diameters b1 were calculated using the obtained AvP values. These deduced hard sphere values were found to be larger than the physical size of the molecule. This result was found to be in general agreement with other investigation in which long range forces (dipole-dipole) dominate.</p>Effects of Discharge Tube Geometry on Plasma Ion Oscillations2015-06-24T09:39:17-05:00https://digital.library.unt.edu/ark:/67531/metadc663636/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc663636/"><img alt="Effects of Discharge Tube Geometry on Plasma Ion Oscillations" title="Effects of Discharge Tube Geometry on Plasma Ion Oscillations" src="https://digital.library.unt.edu/ark:/67531/metadc663636/small/"/></a></p><p>This study considers the effect, on plasma ion oscillations, of various lengths of discharge tubes as well as various cross sections of discharge tubes. Four different gases were used in generating the plasma. Gas pressure and discharge voltage and current were varied to obtain a large number of signals.
A historical survey is given to familiarize the reader with the field. The experimental equipment and procedure used in obtaining data is given. An analysis of the data obtained is presented along with possible explanations for the observed phenomena. Suggestions for future study are made.</p>Relaxation Time Measurements for Collision Processes in the Surface Layers of Conductors and Semiconductors Near 10 Ghz2015-06-24T09:39:17-05:00https://digital.library.unt.edu/ark:/67531/metadc663566/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc663566/"><img alt="Relaxation Time Measurements for Collision Processes in the Surface Layers of Conductors and Semiconductors Near 10 Ghz" title="Relaxation Time Measurements for Collision Processes in the Surface Layers of Conductors and Semiconductors Near 10 Ghz" src="https://digital.library.unt.edu/ark:/67531/metadc663566/small/"/></a></p><p>This thesis represents one phase of a joint effort of research on the properties of liquids and solids. This work is concerned primarily with the microwave properties of solids. In this investigation the properties exhibited by conductor and semiconductor materials when they are subjected to electromagnetic radiation of microwave frequency are studied. The method utilized in this experiment is the perturbation of a resonant cavity produced by introduction of a cylindrically shaped sample into it.</p>A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504009/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504009/"><img alt="A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling" title="A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling" src="https://digital.library.unt.edu/ark:/67531/metadc504009/small/"/></a></p><p>This thesis relates a technique for increasing the optical strength of NaCl and KCl single-crystal samples. The 1.06-μm pulsed laser damage thresholds were increased by factors as large as 4.6 for a bulk NaCl single-crystal sample. The bulk laser damage breakdown threshold (LDBT) of the crystal was measured prior to and after heat treatment at 800*C using a Nd:YAG laser operating at 1.06 μm. Bulk and surface LDBTs were also studied on samples annealed at 400° C. These samples showed differences in damage morphology on both cleaved and polished surfaces, and the cleaved surfaces had improved damage thresholds. However, neither the polished surfaces nor the bulk showed improved threshold at the lower annealing temperature.</p>K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504034/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504034/"><img alt="K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV" title="K-Shell Ionization Cross Sections of Selected Elements from Ag to La for Proton Bombardment from 0.6 to 2.0 MeV" src="https://digital.library.unt.edu/ark:/67531/metadc504034/small/"/></a></p><p>The K-shell x-ray and ionization cross sections are measured for protons on Ag, Cd, Sn, Sb, Te, Ba, and La over the ion energy range of 0.6 to 2.0 MeV. The data are compared to the predictions of the PWBA, the PWBA with corrections for binding energy and/or Coulomb deflection, the BEA, and the constrained BEA predictions. In general, the non-relativistic PWBA with binding energy correction gives the best overall agreement with the measurements of proton-induced x-ray processes for the K-shell of the elements studied in this work. The data further suggest the need for relativistic PWBA treatment of the interactions in the K-shell for the range of binding energies represented by the elements investigated in this work.</p>Automatic Frequency Control of Microwave Radiation Sources2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504304/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504304/"><img alt="Automatic Frequency Control of Microwave Radiation Sources" title="Automatic Frequency Control of Microwave Radiation Sources" src="https://digital.library.unt.edu/ark:/67531/metadc504304/small/"/></a></p><p>Resonant cavity controlled klystron frequency stabilization circuits and quartz-crystal oscillator frequency stabilization circuits were investigated for reflex klystrons operating at frequencies in the X-band range. The crystal oscillator circuit employed achieved better than 2 parts in 10 in frequency stability. A test of the functional properties of the frequency standard was made using the Stark effect in molecules.</p>A Calculation of the Excitation Spectrum of Superfluid Helium-42015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504576/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504576/"><img alt="A Calculation of the Excitation Spectrum of Superfluid Helium-4" title="A Calculation of the Excitation Spectrum of Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc504576/small/"/></a></p><p>The Hartree-Fock-Bogoliubov theory of homogeneous boson systems at finite temperatures is rederived using, a free energy variational principle. It is shown that a t-matrix naturally emerges in the theory. Phenomenological modifications are made (1) to remove the energy gap at zero momentum, and (2) to eliminate the Hartree-Fock-like terms, which dress the kinetic energy of the particle. A numerical calculation of the energy spectrum is made over a temperature range of 0.00 to 3.14 K using the Morse dipole-dipole-2 potential and the Frost-Musulin potential. The energy spectrum of the elementary excitations is calculated self-consistently. It has a phonon behavior at low momentum and a roton behavior at higher momentum, so it is in qualitative agreement with the observed energy spectrum of liquid He II. However, the temperature dependence of the spectrum is incorrectly given. At the observed density of 0.0219 atoms A-3, the depletion of the zero-momentum state at zero temperature is 40.5% for the Morse dipole-dipole-2potential, and 43.2% for the Frost- Musulin potential. The depletion increases gradually until at 3.14 K the zero momentum density becomes zero discontinuously, which indicates a transition to the ideal Bose gas.</p>The Classical Limit of Quantum Mechanics2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504591/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504591/"><img alt="The Classical Limit of Quantum Mechanics" title="The Classical Limit of Quantum Mechanics" src="https://digital.library.unt.edu/ark:/67531/metadc504591/small/"/></a></p><p>The Feynman path integral formulation of quantum mechanics is a path integral representation for a propagator or probability amplitude in going between two points in space-time. The wave function is expressed in terms of an integral equation from which the Schrodinger equation can be derived. On taking the limit h — 0, the method of stationary phase can be applied and Newton's second law of motion is obtained. Also, the condition the phase vanishes leads to the Hamilton - Jacobi equation. The secondary objective of this paper is to study ways of relating quantum mechanics and classical mechanics. The Ehrenfest theorem is applied to a particle in an electromagnetic field. Expressions are found which are the hermitian Lorentz force operator, the hermitian torque operator, and the hermitian power operator.</p>Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504123/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504123/"><img alt="Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors" title="Shubnikov-de Haas Effect Under Uniaxial Stress: A New Method for Determining Deformation Potentials and Band Structure Information in Semiconductors" src="https://digital.library.unt.edu/ark:/67531/metadc504123/small/"/></a></p><p>The problem with which this investigation is concerned is that of demonstrating the applicability of a particular theory and technique to two materials of different band structure, InSb and HgSe, and in doing so, determining the deformation potentials of these materials. The theory used in this investigation predicts an inversion-asymmetry splitting and an anisotropy of the Fermi surface under uniaxial stress. No previous studies have ever verified the existence of an anisotropy of the Fermi surface of semiconductors under stress. In this work evidence will be given which demonstrates this anisotropy. Although the inversion-asymmetry splitting parameter has been determined for some materials, no value has ever been reported for InSb. The methods presented in this paper allow a value of the splitting parameter to be determined for InSb.</p>Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504158/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504158/"><img alt="Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field" title="Test of Gauge Invariance: Charged Harmonic Oscillator in an Electromagnetic Field" src="https://digital.library.unt.edu/ark:/67531/metadc504158/small/"/></a></p><p>The gauge-invariant formulation of quantum mechanics is compared to the conventional approach for the case of a one-dimensional charged harmonic oscillator in an electromagnetic field in the electric dipole approximation. The probability of finding the oscillator in the ground state or excited states as a function of time is calculated, and the two approaches give different results. On the basis of gauge invariance, the gauge-invariant formulation of quantum mechanics gives the correct probability, while the conventional approach is incorrect for this problem. Therefore, expansion coefficients or a wave function cannot always be interpreted as probability amplitudes. For a physical interpretation as probability amplitudes the expansion coefficients must be gauge invariant.</p>Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe2015-05-10T06:16:59-05:00https://digital.library.unt.edu/ark:/67531/metadc504294/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc504294/"><img alt="Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe" title="Microwave Properties of Liquids and Solids, Using a Resonant Microwave Cavity as a Probe" src="https://digital.library.unt.edu/ark:/67531/metadc504294/small/"/></a></p><p>The frequency shifts and Q changes of a resonant microwave cavity were utilized as a basis for determining microwave properties of solids and liquids. The method employed consisted of varying the depth of penetration of a cylindrical sample of the material into a cavity operating in the TM0 1 0 Mode. The liquid samples were contained in a thin-walled quartz tube. The perturbation of the cavity was achieved by advancing the sample into the cavity along the symmetry axis by employing a micrometer drive appropriately calibrated for depth of penetration of the sample. A differentiation method was used to obtain the half-power points of the cavity resonance profile at each depth of penetration. The perturbation techniques for resonant cavities were used to reduce the experimental data obtained to physical parameters for the samples. The probing frequency employed was near 9 gHz.</p>Room Temperature Gold-Vacuum-Gold Tunneling Experiments2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500464/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500464/"><img alt="Room Temperature Gold-Vacuum-Gold Tunneling Experiments" title="Room Temperature Gold-Vacuum-Gold Tunneling Experiments" src="https://digital.library.unt.edu/ark:/67531/metadc500464/small/"/></a></p><p>An experiment has been completed which demonstrated quantum mechanical tunneling of electrons between two gold electrodes separated in vacuum. The tunneling current between the gold electrodes has been measured, for fixed voltages of 0.1 and 0.01 volts, as the electrode spacing was varied from a distance of approximately 2.0 nm down to a point where the electrodes touched. Current-voltage characteristics for fixed electrode spacing in the direct tunneling region have also been measured. Numerical calculations of the tunneling current based on the free-electron model of the electrodes and the barrier, an image-potential reduced barrier, and a WKB approximation for the tunneling probability have been performed and compared with Simmons' theory and with the experimental results. Within experimental error the results indicate that an image potential reduced barrier with the modifications suggested by Lang and Kohn gives a close approximation to the true barrier for metal-vacuum-metal tunneling. For the first time, the work function of the electrodes in a tunneling experiment has been deduced from experimental parameters independent of the tunneling device.</p>Theoretical Cross Section for Light Scattering from Superfluid Helium-42015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500357/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500357/"><img alt="Theoretical Cross Section for Light Scattering from Superfluid Helium-4" title="Theoretical Cross Section for Light Scattering from Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc500357/small/"/></a></p><p>The finite lifetime of the bound roton pair is included in the theoretical light scattering cross section to explain the shape of the peak in the observed Raman light scattering cross section in He II. A model Hamiltonian is used to describe interactions between quasiparticles for the helium system. The equation of motion for the bound roton pair state, which is taken to be a collective mode of quasiparticle pairs, is solved. The cross section for light scattering is then derived using Fermi's Golden Rule with the bound roton pair as the final state. Since the bound roton pair can decay into two free phonons, a phenomenological width r is included in the cross section. The peak position and shape of the observed cross section are both fitted using a binding energy of εB = 0.37 K for the bound roton pair.</p>A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-42015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc501017/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc501017/"><img alt="A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-4" title="A Theoretical Investigation of Bound Roton Pairs in Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc501017/small/"/></a></p><p>The Bogoliubov theory of excitations in superfluid helium is used to study collective modes at zero temperature. A repulsive delta function shell potential is used in the quasiparticle excitation energy spectrum to fit the observed elementary excitation spectrum, except in the plateau region. The linearized equation of motion method is used to obtain the secular equation for a collective mode consisting of a linear combination of one and two free quasiparticles of zero total momentum. It is shown that in this case for high-lying collective modes, vertices involving three quasiparticles cancel, and only vertices involving four quasiparticles are important. A decomposition into various angular momentum states is then made. Bound roton pairs in the angular momentum D-state observed in light-scattering experiments exist only for an attractive coupling between helium atoms in this oversimplified model. Thus, the interaction between particles can be reinterpreted as a phenomenological attractive coupling between quasiparticles, in order to explain the Raman scattering from bound roton pairs in superfluid helium.</p>A Study of L-Shell X-Ray Production Cross Sections Due to [Hydrogen-1], [Helium-4], and [Lithium-7] Ion Bombardment of Selected Thin Rare Earth and ₈₂Pb Targets2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500949/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500949/"><img alt="A Study of L-Shell X-Ray Production Cross Sections Due to [Hydrogen-1], [Helium-4], and [Lithium-7] Ion Bombardment of Selected Thin Rare Earth and ₈₂Pb Targets" title="A Study of L-Shell X-Ray Production Cross Sections Due to [Hydrogen-1], [Helium-4], and [Lithium-7] Ion Bombardment of Selected Thin Rare Earth and ₈₂Pb Targets" src="https://digital.library.unt.edu/ark:/67531/metadc500949/small/"/></a></p><p>Thin target L-Shell x-ray production cross sections for protons incident on ₆₂Sm and ₇₀Yb in the energy range of 0.3 to 2.4 MeV/amu, alpha particles incident on ₆₂Sm, ₇₀Yb, and ₈₂Pb in the energy range of 0.15 to 4.8 MeV/amu, and lithium ions incident on ₅₈Ce, ₆₀Nd, ₆₂Sm, ₆₆Dy, ₆₇Ho, ₇₀Yb, and ₈₂Pb in the energy range of 0.8 to 4.4 MeV/amu have been measured. The cross section data have been compared to the planewave Born approximation (PWBA) and the PWBA modified to include binding energy and Coulomb deflection effects. The Lα₁,₂ x-ray production cross sections are best represented by the PWBA modified to include both the binding energy and Coulomb deflection effects (PWBA-BC) over the entire incident ion, incident energy, and target ranges studied. However, the Lγ₁ and Lγ₂,₃,₍₆₎ x-ray production cross sections are best represented by the PWBA except at the lower ion energies, where both the PWBA and PWBA-BC are in disagreement with the data. The comparison of Lα₁,₂/Lγ₂,₃,₍₆₎ ratios to theory reveals that the PWBA-BC does not predict the inflection point substantiated by the data, and the agreement between the data and the PWBA-BC becomes worse as the atomic number of the incident ion increases. Comparison of the PWBA modified to include binding energy effects CPWBA-B) and the PWBA modified to include Coulomb deflection effects (PWBA-C) to the Lα₁,₂, Lγ₁, and the Lγ₂,₃ cross sections for protons, alpha particles, and lithium ions incident on ₇₀Yb indicates that the PWBA-C overestimates the magnitude of the data but does describe the shape of the L₁-associated cross section while the PWBA-B underestimates the magnitude of the data but fails to predict the proper shape of the L₁-associated data. In order to evaluate the ability of the PWBA and the presently accepted modifications to the PWBA to fit the experimental data, future experimentation should be conducted in the energy range that includes the point where the ratio of the incident ion velocity to the L-Shell electron velocity is equal to 0.19 (i.e., V₁/Vₗ = 0.19). This is where the L₁-associated cross sections begin to exhibit the shouldered structure and the cross section ratios L₃/L₁ and L₂/L₁ have inflection points.</p>K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500606/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500606/"><img alt="K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV" title="K-Shell Ionization Cross Sections of Selected Elements from Fe to As for Proton Bombardment from 0.5 to 2.0 MeV" src="https://digital.library.unt.edu/ark:/67531/metadc500606/small/"/></a></p><p>The problem with which this investigation is concerned is that of making experimental measurements of proton-induced K-shell x-ray production cross sections and to study the dependence of these cross sections upon the energy of the incident proton. The measurements were made by detection of the characteristic x-rays emitted as a consequence of the ionization of the K-shell of the atom. The method for relating this characteristic x-ray emission to the x-ray production cross section is discussed in this work.</p>The Shubnikov-de Haas Effect in N-Type Indium Antimonide2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500637/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500637/"><img alt="The Shubnikov-de Haas Effect in N-Type Indium Antimonide" title="The Shubnikov-de Haas Effect in N-Type Indium Antimonide" src="https://digital.library.unt.edu/ark:/67531/metadc500637/small/"/></a></p><p>The Shubnikov-de Haas effect is an oscillation in the electrical resistivity or conductivity of a metal, semimetal, or semiconductor as a function of changing magnetic field which occurs at low temperatures. The effect is caused by the quantization of the momentum and energy of the charge carriers by the magnetic field. Since the nature of the oscillation depends strongly on the energy band structure of the material in which it is measured, the effect could be quite useful as an investigative tool. Its usefulness has been limited, however, by the uncertainty as to the functional form of the relationship between the measured oscillations and the parameters characterizing the material. One purpose of the present study is to extend the usefulness of the Shubnikov-de Haas effect by experimentally determining the functional form appropriate for a material such as n-type indium antimonide. The second purpose of the study is to determine values for the parameters which characterize the band structure of indium antimonide. The curve fitting procedure is found to be a powerful tool for investigating band structure. All computer programs used in processing the data, fitting the data, and comparing the results with the Kane model are given.</p>Density Profile of a Quantized Vortex Line in Superfluid Helium-42015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500722/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500722/"><img alt="Density Profile of a Quantized Vortex Line in Superfluid Helium-4" title="Density Profile of a Quantized Vortex Line in Superfluid Helium-4" src="https://digital.library.unt.edu/ark:/67531/metadc500722/small/"/></a></p><p>The density amplitude of an isolated quantum vortex line in superfluid 4He is calculated using a generalized Gross-Pitaevskii (G-P) equation. The generalized G-P equation for the order parameter extends the usual mean-field approach by replacing the interatomic potential in the ordinary G-P equation by a local, static T matrix, which takes correlations between the particles into account. The T matrix is a sum of ladder diagrams appearing in a diagrammatic expansion of the mean field term in an exact equation for the order parameter. It is an effective interaction which is much softer than the realistic interatomic Morse dipole-dipole potential from which it is calculated. A numerical solution of the generalized G-P equation is required since it is a nonlinear integro-differential equation with infinite limits. For the energy denominator in the T matrix equation, a free-particle spectrum and the observed phonon-roton spectrum are each used. For the fraction of particles in the zero-momentum state (Bose-Einstein dondensate) which enters the equation, both a theoretical value of 0.1 and an experimental value of 0.024 are used. The chemical potential is adjusted so that the density as a function of distance from the vortex core approaches the bulk density asymptotically. Solutions of the generalized G-P equation are not very dependent on the choice of energy denominator or condensate fraction. The density profile is a monotonically increasing function of the distance from the vortex core. The core radius, defined to be the distance to half the bulk density, varies from 3.7 A to 4.7 A, which is over three times the experimental value of 1.14 A at absolute zero.</p>K-Shell Ionization Cross Sections For Elements Se To Pd: 0.4 To 2.0 MeV2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500564/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500564/"><img alt="K-Shell Ionization Cross Sections For Elements Se To Pd: 0.4 To 2.0 MeV" title="K-Shell Ionization Cross Sections For Elements Se To Pd: 0.4 To 2.0 MeV" src="https://digital.library.unt.edu/ark:/67531/metadc500564/small/"/></a></p><p>K-Shell ionization cross section for protons over the energy range of 0.4 to 2.0 MeV have been measured on thin targets of the elements Se, Br, Rb, Sr, Y, Mo and Pd. Total x-ray and ionization cross sections for the K-shell are reported. The experimental values of the ionization cross sections are compared to the non-relativistic plane-wave Born approximation, the binary-encounter approximation, the constrained binary-encounter approximation, and the plane-wave Born approximation with corrections for Coulomb-deflection and binding energy effects.</p>The Role of Defects in the Quantum Size Effect2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500650/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500650/"><img alt="The Role of Defects in the Quantum Size Effect" title="The Role of Defects in the Quantum Size Effect" src="https://digital.library.unt.edu/ark:/67531/metadc500650/small/"/></a></p><p>This investigation is a theoretical study of the influence of defects of finite volume on the electrical conductivity in the quantum size effect regime. Correction terms to existing equations are derived, and a physical explanation of the results is given. Many macroscopic properties of films exhibit an oscillatory dependence on thickness when the thickness is comparable to the de Broglie wavelength of an electron at the Fermi surface. This behavior is called the quantum size effect. In very thin films, scattering from surfaces, phonons, and crystal defects plays an increasingly important role. In this investigation the influence of scattering centers (defects) in semimetal films on the electrical conductivity is explored by extending existing work to include scattering centers of finite range. The purpose of this study is to determine the overall change in the conductivity and the alteration of the amplitude of the oscillations. The Boltzmann transport equation is the starting point for the calculation. An equation for the vector mean free path is derived, and a solution is obtained by the iterative process. The relaxation approximation need not be made since the vector mean free path is determined. The sample is a thin slab that is infinite in two dimensions. The assumption is made that the electron wave function is zero at the walls of the sample. It is further assumed that there is a known number of randomly located defects within the slab. The noninteracting electrons are considered free except in the vicinity of the scattering centers. The defects are characterized by a potential that is constant within a small cube and zero outside of it. This approach allows the potential matrix elements to be evaluated by expanding in a power series. The electrical conductivity is calculated for three defect sizes, and a comparison is made to 3-function (infinitely small) scattering centers. An overall decrease in the conductivity is found in each case, and the absolute magnitude of the oscillations is decreased. The percentage of oscillation, however, is increased. The general conductivity decrease is attributed to the increase in the scattering range. The change in the amplitude of the oscillations is explained by analyzing the transition probabilities to available energy states at critical film thicknesses. The oscillations are found to be a result of transitions from states with large energies in the plane of the film to states with small energies in the plane of the film. The number of electrons occupying the various states is determined at critical film thicknesses, and a comparison with the conductivity equation shows excellent agreement.</p>Collision Broadening of Microwave Spectral Lines of Monomeric Formaldehyde and Formic Acid2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500522/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500522/"><img alt="Collision Broadening of Microwave Spectral Lines of Monomeric Formaldehyde and Formic Acid" title="Collision Broadening of Microwave Spectral Lines of Monomeric Formaldehyde and Formic Acid" src="https://digital.library.unt.edu/ark:/67531/metadc500522/small/"/></a></p><p>Line width parameters for a number of spectral lines in the pure rotational spectrum of formaldehyde (CH20) and formic acid (HCOOH) have been measured using a sourcemodulated microwave spectrograph. All transitions studied in this investigation were of the type ΔJ=O (i.e. Q-branch transitions), with ΔK-1=0 and ΔK+1 =+l. The center frequencies of the measured lines varied from 8662.0 MHz to 48612.70 MHz. The experimentally determined collision diameters for self broadening interactions involving HCOOH and CH2 Q molecules were found to be 2 - 27 per cent less than those calculated by the Murphy-Boggs theory of collision broadening. Much better agreement between a simplified broadening scheme for symmetric top molecules and the observed foreign-gas collision diameters is obtained by using Birnbaum's theory.</p>A Classical Theory of the Dielectric Susceptibility of Anharmonic Crystals2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc501119/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc501119/"><img alt="A Classical Theory of the Dielectric Susceptibility of Anharmonic Crystals" title="A Classical Theory of the Dielectric Susceptibility of Anharmonic Crystals" src="https://digital.library.unt.edu/ark:/67531/metadc501119/small/"/></a></p><p>An expression for the dielectric susceptibility tensor of a cubic ionic crystal has been derived using the classical Liouville operator. The effect of cubic anharmonic forces is included as a perturbation on the harmonic crystal solution, and a series expansion for the dielectric susceptibility is developed. The most important terms in the series are identified and summed, yielding an expression for the complex susceptibility with an anharmonic contribution which is linearly dependent on temperature. A numerical example shows that both the real and imaginary parts of the susceptibility are continuous, finite functions of frequency.</p>L X-Ray Production in the Rare Earths by 0.33-2.66-MeV/amu Carbon- and 0.50-2.25-MeV/amu Oxygen-Ion Bombardment2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500238/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500238/"><img alt="L X-Ray Production in the Rare Earths by 0.33-2.66-MeV/amu Carbon- and 0.50-2.25-MeV/amu Oxygen-Ion Bombardment" title="L X-Ray Production in the Rare Earths by 0.33-2.66-MeV/amu Carbon- and 0.50-2.25-MeV/amu Oxygen-Ion Bombardment" src="https://digital.library.unt.edu/ark:/67531/metadc500238/small/"/></a></p><p>Experimentally measured L-shell x-ray production cross sections are presented for 8-36-MeV oxygen-ion bombardment of Ce, Pr, Sm, Eu, Dy, and Ho; for 4-32-MeV carbon-ion bombardment of La and Yb; for 6-32-MeV carbon-ion bombardment of Pr, Nd, Sm, and Dy; and for ll-29-MeV carbon-ion bombardment of Ce, Eu, Gd, and Ho. Theoretical predictions via the plane wave Born approximation (PWBA) with corrections for increased binding of target electrons and Coulomb deflection of the incident projectile tend to underestimate the experimental data; and this underestimation tends to get worse at the low- and high-energy ends of the range of energies used in this work.</p>Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500274/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500274/"><img alt="Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde" title="Collision Broadening in the Microwave Rotational Spectrum of Gaseous Monomeric Formaldehyde" src="https://digital.library.unt.edu/ark:/67531/metadc500274/small/"/></a></p><p>A source-modulation microwave spectrograph was utilized to measure line width parameters for several spectral lines in the pure rotational spectrum of formaldehyde (H₂CO). The spectrograph featured high-gain ac amplification and phase-sensitive detection, and was capable of measuring microwave lines having absorption coefficients as small as 10⁻⁷ cm⁻¹ with a frequency resolution on the order of 30 kHz. Center frequencies of the measured lines varied from 4,830 MHz to 72,838 MHz; hence, most of the observations were made on transitions between K-doublets in the rotational spectrum. Corrections were applied to the measured line width parameters to account for Doppler broadening and, where possible, for deviations due to magnetic hyperfine structure in some of the K-doubled lines. Low modulation voltages and low microwave power levels were used to minimize modulation and saturation broadenings; other extraneous broadenings were found to be insignificant. The primary broadening mechanism at low gas pressure is pressure broadening, and a review of this topic is included. Line width parameters for the several observed transitions were determined by graphing half-widths versus pressure for each spectral line, and performing a linear least-squares fit to the data points. Repeatability measurements indicated the accuracy of the line width parameters to be better than ±10 percent. The reasons for this repeatability spread are discussed, Broadening of each line was measured for self- and foreign-gas broadening by atomic helium and diatomic hydrogen. Effective collision diameters were calculated for each broadening interaction, based on the observed rates of broadening.</p>Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy2015-03-09T08:15:06-05:00https://digital.library.unt.edu/ark:/67531/metadc500284/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500284/"><img alt="Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy" title="Proton-Induced L-shell X-Rays of Pr, Sm, Eu, Gd, and Dy" src="https://digital.library.unt.edu/ark:/67531/metadc500284/small/"/></a></p><p>Characteristic L-shell x rays of the five rare earths Pr, Sm, Eu, Gd, and Dy were studied in this work. The x rays were produced by ionization from 0.3 to 2.0 MeV protons from the 2.0 MV Van de Graaff at North Texas State University. Total L-shell ionization and x-ray production cross sections were measured for Sm and compared to the BEA, CBEA and PWBA theories. Total L-shell ionization cross sections were measured for Pr, Eu, Gd, and Dy and compared to the BEA, CBEA, and PWBA. The CBEA and PWBA fit the samarium data well for both ionization and x-ray production cross sections. The BEA was generally 40 per cent lower than the data. The CBEA and the PWBA also fit the ionization cross section data for Pr, Eu, Gd and Dy, while the BEA was generally 40 per cent lower than the data.</p>