Open Access. Powered by Scholars. Published by Universities.®
Articles 1 - 9 of 9
Full-Text Articles in Entire DC Network
Evolutionary Effects Of Irradiation In Cataclysmic Variables., Patrick Joseph Mccormick
Evolutionary Effects Of Irradiation In Cataclysmic Variables., Patrick Joseph Mccormick
LSU Historical Dissertations and Theses
We developed a simple stellar computational model consisting of two concentric polytropic shells in order to investigate compact binary evolution. In this thesis we focus the investigation on the effects of irradiation on the orbital evolution of cataclysmic variables (CVs). In these systems, when the companion is illuminated by a fraction of the accretion luminosity, the orbital evolution consists of irradiation-driven limit cycles on thermal timescales. These cycles are superimposed on the secular evolution toward shorter periods due to systemic angular momentum losses. When the irradiation instability is enhanced by consequential angular momentum losses $j\sb{\rm CAML}$ the net effect is …
Photoionization Studies Of Intramolecular Dynamics: A Closer Look., Chuanyong Wu
Photoionization Studies Of Intramolecular Dynamics: A Closer Look., Chuanyong Wu
LSU Historical Dissertations and Theses
Using synchrotron radiation as an excitation source, dispersed fluorescence measurements of two different molecular systems are presented in this dissertation with the intention to study the two fundamental aspects of shape resonances-complexity and completeness. $\rm C\sb6F\sb6$ is a relative large and complex molecule in the sense that nonradiative transitions can take place unimolecularly in the free species. The elucidation of the ionization dynamics of this molecule helps to bridge the gap from simple molecules to matter in condensed phases. In the $\rm 2a\sb{2u}\sp{-1}$ photoionization study of $\rm C\sb6F\sb6,$ the non-Franck-Condon behavior of the vibrational distributions at around 20 eV indicates …
Magnetic Field Evolution In Neutron Stars., Erik Jordan Young
Magnetic Field Evolution In Neutron Stars., Erik Jordan Young
LSU Historical Dissertations and Theses
Magnetized neutron stars provide the basic model for radio pulsars. They also play a fundamental role in the theory of pulsating X-ray sources and explaining the origin of millisecond radio pulsars. In an effort to better understand these interesting and fundamentally important astrophysical systems, this dissertation studies the magnetic evolution of neutron stars with a dipole field supported in the crust. We explore their evolution both as isolated systems and as accreting objects in binary systems. We investigate how the incorporation of the gravitational field of the neutron star core into the spacetime occupied by a dipole field in the …
Pressure-Induced Changes In Structural And Mechanical Properties Of Gallium Arsenide And Nanophase Silicon Carbide Using Parallel Molecular Dynamics Simulations., Alok Chatterjee
LSU Historical Dissertations and Theses
Large-scale parallel molecular-dynamics (MD) simulations are performed to investigate properties and processes in nanophase silicon carbide (n-SiC) and gallium arsenide (GaAs) under pressure. These simulations involve 10 6 atoms interacting via realistic interatomic potentials. Structure and sintering of crystalline n-SiC are also investigated with neutron scattering experiments performed at the Intense Pulsed Neutron Source Division (IPNS), Argonne National Laboratory. Both MD simulations and neutron scattering experiments indicate the onset of sintering around 1500K. During sintering, the pores shrink in size while maintaining their morphology: Their fractal dimension remains ∼2.4 and their surface roughness exponent is ∼0.45. The pair distribution functions …
The Structure And Stability Of Selected, 2-D Self-Gravitating Systems., Saied W. Andalib
The Structure And Stability Of Selected, 2-D Self-Gravitating Systems., Saied W. Andalib
LSU Historical Dissertations and Theses
Models of radially and vertically extended self-gravitating disks orbiting around a central point mass are relevant to the dynamics of astrophysical systems and are thought to be common in many galaxies. The gravity driven instabilities in these accretion disks are now believed to be a possible mechanism for star formation via disk fragmentation (Shu, Adams, & Lizano 1987, Adams, Rudin & Shu 1989; Christodoulou 1995). We quantify these regions of instability using a simple toroidal model of an accretion disk. We choose the two-dimensional axisymmetric, incompressible slender disks to examine and map out these principal modes of gravity driven instabilities. …
The Selective Digital Integrator: A New Device For Modulated Polarization Spectroscopy., Aljosa Vrancic
The Selective Digital Integrator: A New Device For Modulated Polarization Spectroscopy., Aljosa Vrancic
LSU Historical Dissertations and Theses
A new device, a selective digital integrator (SDI), for the acquisition of modulated polarization spectroscopy (MPS) signals is described. Special attention is given to the accurate measurement of very small (AC component of interest $<$10$\sp{-3}$ x DC component), rapidly modulated ($\sim$50 kHz) signals at or below noise levels. Various data acquisition methods and problems associated with the collection of modulated signals are discussed. The SDI solves most of these problems and has the following advantages: it provides the average-time resolved profile of a modulated signal; it eliminates errors if the modulation is not sinusoidal; it enables separate measurements of the various phases of the signal modulation cycle; it permits simultaneous measurement of absorption, circular dichroism (CD) and linear dichroism (LD) spectra; it facilitates 3-D absorbance measurements; it has a wide gain-switching-free dynamic range (10 orders of magnitude or more); it offers a constant S/N ratio mode of operation; it eliminates the need for photomultiplier voltage feedback, and it has faster scanning speeds. The time-resolution, selectivity, wide dynamic range, and low-overhead on-the-fly data processing are useful for other modulated spectroscopy (MS) and non-MS experiments such as pulse height distribution and time-resolved pulse counting measurements. The advantages of the MPS-SDI method are tested on the first Rydberg electronic transitions of (+)-3-methylcyclopentanone. The experimental results validate the predicted SDI capabilities. However, they also point to two difficulties that had not been noted previously: the presence of LD in a gaseous sample and a pressure-dependence of the relative peak heights of the CD spectrum. Models for these anomalies are proposed. The presence of the oscillatory LD (but not an LD background) is explained with a sample cell model based on the observed polarization-dependent time-resolved profiles of transmitted light intensity. To obtain expressions for these intensities, a theoretical background, which provides a new approach to the treatment of light/matter interaction, is included as an Appendix. To explain the second anomaly, present only at high optical densities, a model based on the presence of scattered light is introduced and verified. The mode of correction for the scattering problem is outlined.
Nanostructure Modeling In Oxide Ceramics Using Large Scale Parallel Molecular Dynamics Simulations., Timothy J. Campbell
Nanostructure Modeling In Oxide Ceramics Using Large Scale Parallel Molecular Dynamics Simulations., Timothy J. Campbell
LSU Historical Dissertations and Theses
The purpose of this dissertation is to investigate the properties and processes in nanostructured oxide ceramics using molecular-dynamics (MD) simulations. These simulations are based on realistic interatomic potentials and require scalable and portable multiresolution algorithms implemented on parallel computers. The dynamics of oxidation of aluminum nanoclusters is studied with a MD scheme that can simultaneously treat metallic and oxide systems. Dynamic charge transfer between anions and cations which gives rise to a compute-intensive Coulomb interaction, is treated by the O(N) Fast Multipole Method. Structural and dynamical correlations and local stresses reveal significant charge transfer and stress variations which cause rapid …
Measurement Of The Cross Section For Elastic Scattering Of Electron Neutrinos On Electrons., Neville Dinyar Wadia
Measurement Of The Cross Section For Elastic Scattering Of Electron Neutrinos On Electrons., Neville Dinyar Wadia
LSU Historical Dissertations and Theses
In this dissertation, we measure the cross section for the elastic scattering of electron neutrinos on electrons. We use data from the LSND experiment which is located at the Los Alamos Neutron Scattering Center at Los Alamos National Laboratory, New Mexico. The neutrino beam is produced when an 800 MeV proton beam from a linear accelerator is incident on a target located 29.8 m from the detector. The LSND veto system allows us to reject charged cosmic-ray particles entering the detector with high efficiency. The detector consists of 180 tons of mineral oil, to which a small quantity of scintillator …
M1 Strength Distributions In Deformed Nuclei., Thomas Georg Beuschel
M1 Strength Distributions In Deformed Nuclei., Thomas Georg Beuschel
LSU Historical Dissertations and Theses
The Elliott SU(3) Model, extended via pseudo-spin for heavy nuclei, is used to study low-lying magnetic dipole excitations in deformed nuclei which axe known as "scissors" modes. Proton and neutron degrees of freedom are handled explicitly and a system Hamiltonian that preserves SU(3) symmetry and one that includes single particle energies as well as quadrupole-quadrupole and pairing two-body interactions are considered. Starting from a basic nuclear Hamiltonian that preserves SU(3) symmetry, a microscopic interpretation of the "scissors" mode of the Two Rotor Model is realized through a linear mapping between invariants of the rotor group and SU(3). The model allows …