Date of Award

Fall 1998

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Reid, Scott A.

Second Advisor

Hossenlopp, Jeanne

Abstract

To understand the chemistry of many important processes, such as combustion, explosion and chemical vapor deposition, it is necessary to have probes for the unstable, highly reactive chemical species involved in these processes as reactants or intermediates. Effective approaches to these short-lived, energetic molecules or radicals demand sensitive, universal, and proficient techniques. During the last decade, some new linear and nonlinear methods, such as Cavity-Ring-Down Spectroscopy (CRDS), Degenerate Four Wave Mixing (DFWM) have been developed as diagnostic techniques alternative to the conventional spectroscopies, and have attracted much attention. Previous work on DFWM has demonstrated strongly its potential as a proficient diagnostic tool owing to the advantageous features, such as general applicability, accessibility to hostile environments. However, due to the inherent complexity, the practical, applications are subject to certain limitations and difficulties. Therefore, many efforts are still ongoing, both experimentally and theoretically, to achieve a thorough understanding of this method. The high detection limit and general applicability of CRDS have been shown in a number of spectroscopic studies. This method has been proven to be reliable under condition when Beer's law is held. In practice, its applicapility, particularly in infrared spectral region, is subject to considerations about the relationships between the transition line width, laser spectral width and cavity round-trip time. The work described in this thesis is mainly focused on implementing DFWM and CRDS techniques, and investigating the feasibility of applying them to chemical species with specific spectral features and under certain experimental conditions. In chapter 2, it first describes the implementation of DFWM in infrared spectral region, with the aid of a tunable pulsed radiation source based on the Optical Parametric Amplifier technology, and the extension of IR DFWM to investigation of C2H2 in a supersonic expansion. In the framework of the MCECA and AL models, the DFWM response with Gaussian shaped, broadband excitation sources are examined in both weak field and strong field regimes. Chapter 3 demonstrates the work of multiplexing of DFWM and LIF on jet-cooled N02 as a convenient way to obtain molecular information, which are difficult to access via any single approach. Experiments of time-domain DFWM described in chapter 4 illustrate the applicability of DFWM in investigation of time-dependent processes, such as the molecular velocity distribution (i.e., transverse temperature) and lifetime of the excited state. Embodied with the quantum beat techniques, time-domain DFWM also promises sub-Doppler resolution to probe congested molecular state properties. In chapter 5, the applicability of IR CRDS with broadband excitations on jet-cooled chemical species is investigated.

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