Thesis (Ph.D)--Physics, Georgia Institute of Technology, 2009. / Committee Chair: Li Mo; Committee Member: Chou Meiyin; Committee Member: First Phillip; Committee Member: Nguyen Toan; Committee Member: Zangwill Andrew. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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Understanding the radiolysis of impure water and resulting reactions is crucial to many fields. Reactions driven by low energy electrons (LEE) are of special interest, as high-energy radiation generates large quantities of these electrons, which then provide the energy for most subsequent reactions. Interfacially located reactions are also of particular interest, both as models for heterogeneously distributed reactions occurring during radiolysis, and in their own right, as radiation-driven reactions at interfaces are responsible for key processes such as corrosion and DNA damage. To study LEE-driven reactions at interfaces, thin-layered films of amorphous solid water (ASW) and methanol were grown under ultra-high vacuum conditions using molecular beam techniques. The films were exposed to a beam of low-energy (100eV or less) electrons, and studied using electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). ESD studies indicated that methanol moves through a water film during deposition at 80 K but not at 50 K. This transport was not seen during thermal annealing, but radiation-induced mixing was observed at all temperatures. Major and minor LEE radiation products of pure methanol films were identified and found to be consistent with previous results. Products of LEE irradiated layered methanol/water films were determined for the first time using ESD and TPD spectra, and found to be limited to H₂, O, O₂, CH₂O, C₂H₆, CO, CO₂, CH₃OCH₃, and CH₃CH₂OH. The effect of adding methanol to an ASW film on the production in ASW of H₂ and O₂ was also examined. The interface created by the addition of CH₃OH to ASW was found to generate H₂ in previously non-reactive regions of the water film by increasing water-water and water-methanol reactions. Radiative mixing of CH₃OH and ASW enhanced this effect, presumably by increasing the region of disrupted H-bonding in the ASW. In contrast, the addition of CH₃OH at low coverages suppressed O₂ production in both unprocessed and preprocessed ASW layers. Modeling indicates that methanol scavenging of the O₂ precursor OH and of the reaction-driving electrons is responsible for this reduction in O₂ signal.

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

Amorphous metallic alloys are produced by a variety of techniques some of which involve rapid solidification of the alloying constituents. In these methods the solidification occurs so rapidly that the atoms are frozen-in and partially retain their liquid configuration. There are clear structural and other indications from their various properties that amorphous metallic alloys possess short range order but lack long range order. In general, amorphous alloys are not in a thermodynamic equilibrium state and, therefore, relax structurally whenever atoms attain an appreciable mobility. Associated with structural relaxation, many physical properties change; some significantly and others only slightly. Relaxation experiments in amorphous metallic alloys often display approximate In(t) kinetics which can be understood in terms of various models. In the present work the model by Primak (1955), for which the kinetic behaviour of a system depends on processes that are distributed over a range of activation energies, is used as a basis for further development. The Primak model allows, in principle, for the identification of the order of the relaxation reaction and for the determination of an initial activation energy spectrum Po(Єo), where Єo is a characteristic activation energy. Although the model provides for a qualitative explanation of the In(t) law, it has no predictive power as to the quantitative changes accompanying the various relaxing properties. Furthermore, an estimation of Po( Єo), inferred from various isothermal annealing procedures, reveals the approximate shape but does not fix its location on the activation energy axis. These shortfalls are attributed to complications in the frequency factor v, inherent to the Primak model. Also, the Primak model does not include consideration of the entropy involved in a 'configurational jump' of any particular atom during the relaxation process. Inclusion of the configurational entropy through the frequency factor v, in the present treatment, leads to a 'relaxation equation'. Structural relaxation measurements of density (in practice length - from which density can be approximately inferred) and electrical resistivity, in an Fe4oNi40B20 alloy, have been obtained and fitted to this relaxation equation. The fitting parameters are found, within experimental error, to be the same for both length and resistivity relaxation. The initial activation energy spectrum Po(Єo), as inferred from the fits, over the energy range 1.4 to 2.0 eV, reveals roughly three regimes, namely below 1.5 eV, from 1.5 to 1.8 eV, and above 1.8 eV, respectively, over which the initial activation energy spectrum Po( Єo) assumes different approximately constant values. Previous treatments have, however, implicitly assumed that Po( Єo) is constant throughout a temperature range over which In(t) kinetics is observed. The behaviour observed in this work is associated with the intrinsic relaxation mechanism involving consecutive diffusion of the metallic and metalloid atoms, respectively. A configurational entropy change inferred from this work is found to be negative as a consequence of contraction of the spread-out free volume resulting from thermal fluctuations. Within the framework of the 'present model', other related behaviour of amorphous metallic alloys, including the glass transition, crystallization and diffusion, are discussed. Where direct comparison between theory and experiment is possible for the various observed phenomena, the agreement is good and shows an overall consistency in our approach. Finally, the analysis considered here gives an expression which can be easily used to make quantitative predictions about the experimental relaxation behaviour. An immediate understanding of some of the main features of experimental data on relaxation can, therefore, be obtained through application of the present model. / Thesis (Ph.D.)-University of Natal, Durban, 1998.

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This thesis consists of two completely separate parts: In part I some problems related to phonons in amorphous solids are considered, whilst Part II is devoted to the study of excitons in Cuprous Oxide (Cu₂O). Part I.- A theoretical model, suitable to treat vibrations in tetrahedrally coordinated amorphous systems is developed and permits the study of a number of situations of current interest. Three of these situations are studied in detail: 1) The local response of hydrogen in amorphous silicon when a single silicon is attached to one, two or three hydrogens. The differences between these three configurations are discussed and a direct connection with experimental results is suggested. 2) The interesting case of an amorphous alloy, where both, topological and substitutional disorder are present. This is treated within the spirit of the Coherent Potential Approximation. The particular alloy chosen (Si-Ge) is readily tractable because oithe similar bond characteristics of both components, which allows the neglect of force constant changes. 3) The Raman spectrum of AX₂, glasses. The model adopted permits the investigation of the local response at the defect sites for a number of defects. In order to explain the defect lines observed in the experiments, four plausible defect configurations are considered: a missing A-X bond, a X-A double bond, an A-A bond, and a square ring (two tetrahedra sharing an edge). A simple model to calculate the Raman response in amorphous solids is also outlined. Part II.- The valence band of Cu₂0 is studied in detail to account for the deviations from the hydrogenic law of the exciton spectrum. The appearance of the two series of excitons is explained in terms of a spin-orbit splitting of the valence band in the centre of the Brillouin Zone, using a Tight-Binding Approximation. The deviations of the lowest exciton levels from their expected values are seen to arise from an admixture of the two components of the split -off valence band due to direct Coulomb and exchange interactions. The Hamiltonian used corresponds to the so called "Spherical Approximation" and the results obtained are in remarkable agreement with the experiments.

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Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (p. 98-106).