published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

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The results of cross polarized pump-probe experiments in preformed He plasma waveguides are reported. Pump and probe have same wavelength and duration of 800nm and 80fs respectively. Peak pump intensity is I[subscript guided] = 0.2X10¹⁸W/cm² ~1000 I[subscript probe]. Single shot probe spectra and mode profiles at the channel exit are discriminated from the pump with a polarization analyzer and captured at various relative time delays [Delta]t. Frequency-domain interference (FDI) between the probe and a weak depolarized component of the pump is observed for [scientific equation]. Although the depolarized component is nearly undetectable through measurement of pump leakage alone, FDI sensitively reveals its substantially non-Gaussian structure. The possible depolarization mechanisms are analyzed. When probe is positioned at the leading edge of the pump, [scientific equation], its spectrum suffers a blue shift not measurable in the transmitted pump itself. The evidence suggests the channel interior is fully ionized and the partially formed channel ends are the origin of both depolarization and blue shift. A robust, pulsed, differentially-pumped plasma channel generation cell for high intensity guiding experiments has been developed. The design includes an axicon lens, windows for transverse interferometry, and permits injection of one or two different gases (main gas plus high Z seed gas) with several millisecond injection times and simultaneous 0.1ms pressure sensing resolution. Very well formed plasma waveguides have been formed in helium as well as hydrogen, at repeatable and well controlled pressures up to 1000Torr, with very uniform interior density, rapid density drop at boundaries, and very low exterior density. The possible danger associated with the use of large amounts of hydrogen was considered and a complex safety system was designed, constructed and used. Extensive analysis of channel profile reconstruction through transverse interferometry was performed. This includes an intuitive, efficient reformulation and extension of the Phase Locked Loop (PLL) carrier fringe demodulation method. It is also demonstrated and explained how and under which conditions artificial fringe frequency multiplication can reduce demodulation distortions in both PLL and Fast Fourier Transform (FFT) methods. / text

A numerical method has been developed for solving the complete compressible Navier-Stokes equations. The method is applicable for Direct Numerical Simulations (DNS) and Large-Eddy Simulations (LES) and was used here to study the evolution of three-dimensional disturbances in the laminar and turbulent near wake of axisymmetric bluff bodies with a blunt base in supersonic flows. The main objective of this research is to investigate the time dependent behavior of these disturbances and their influence on and interaction with the global flow field. The equations are solved in a cylindrical coordinate system using finite difference approximations of fourth-order accuracy in axial and radial directions and and a fourth-order accurate explicit Runge-Kutta scheme for the time integration. A pseudo-spectral method is employed in the azimuthal direction. Direct Numerical Simulations (DNS) were performed for a subsonic free stream Mach number of M ͚ = 0.2 and for supersonic free stream Mach numbers of M ͚ = 1.2 and M ͚ = 2.46. Large-Eddy Simulations (LES) were carried out for a subsonic free stream Mach number of M ͚ = 0.2 and a global Reynolds number of ReD = 2,000 and for a supersonic free stream Mach number of M ͚ = 2.46 and global Reynolds numbers of ReD = 30,000 and ReD = 100,000. Comparison of the instantaneous flow field for subsonic calculations with water channel experiments and incompressible simulations show good qualitative agreement. An absolute instability with regard to helical disturbances was found for the subsonic flow at ReD = 1,000 and for the supersonic flows for M ͚ = 1.2 and ReD ≥ 4,000 and for M ͚ = 2.46 and ReD ≥ 30,000. Small disturbances appear in the flow field near the corner of the base. As the disturbances are propagating downstream they grow and form intense vortical structures. These structures have a strong influence on the flow field, which results in a drastic change of the base pressure distribution and thus of the base drag.

The current semiconductor device manufacturing requires more strict control of plasma etching. In this research, plasma etching was investigated through gas phase characterization and interface reactions. Hydrogen and nitrogen were added to Ar plasmas to manipulate the electro-physical properties that were measured by a Langmuir probe system. Hydrogen addition modified the EEDF (electron energy distribution function) by increasing the electrons in high energy range. Adding N2 formed a strong bi-Maxwellian distribution. Gas addition caused the transition between ohmic and stochastic heating. Ar-CH4-H2 and Ar-N2-H2 plasmas were also tested. Hydrogen atom beam was used on porous silicon dioxide based low-k films to remove silanol groups that were generated due to the damage of films during pattern transfer. At H2 atom beam process at 150 C moved close to 60% silanol groups were removed in less than 3 min with an etching rate of 15 A/min. The apparent activation energy was 2.4 kcal/mol. Hydrogen atoms reacted with Si-O-Si and methyl groups. The etching mechanisms of CH4/H2/Ar plasma for InP were analyzed by a beam reactor system. Sputtering yield was measured, threshold energy was approximately 60 eV. Inert ion beam assisted chemical reactions gave higher etching rate. The CH4 concentration had no strong effect on etching rate after 5%. Etching rate was not sensitive to temperature up to 150 C. The adsorption of methyl groups to the surface was proposed as rate limiting step. Chemical reaction effectively reduced the surface roughness.

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Abstract available in PDF file. / Thesis (Ph.D.)-University of Natal, 1979.

Thesis (Ph. D.)--University of Washington, 1989. / Vita. Includes bibliographical references (leaves [136]-137).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.