Investigation of molecular spectroscopy by linear and nonlinear methods
Abstract
To understand the chemistry of many important processes, such as combustion, explosion and chemical vapor deposition, it is necessary to have sensitive, universal, and proficient probes for the highly reactive chemical species involved in these processes. 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. However, due to the inherent complexity, the practical applications of DFWM are subject to certain limitations and difficulties. Therefore, many efforts are still ongoing to achieve a thorough understanding of this method. In practice, the applicability of CRDS, particularly in infrared spectral region, is subject to considerations about the relationships between the transition linewidth, 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. 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 C2 H2 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. The multiplexing of DFWM and LIF on jet-cooled NO2 is demonstrated to be a convenient way to obtain molecular information, which are difficult to access via any single approach. Further experiments of time-domain DFWM 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. At last, the applicability of IR CRDS with broadband excitations on jet-cooled chemical species is investigated.
This paper has been withdrawn.