Development of absorption-based spectroscopic techniques for detection of diatomic and polyatomic radicals in flames

Absorption-based laser diagnostic techniques have been developed and applied in flame environments to measure diatomic and polyatomic transient species. The in situ detection and quantification of radical species improves our understanding of the complex chemical kinetics existing in many combustion systems and provides a data base to aid in the development and evaluation of chemical kinetic models. This research specifically focused on two techniques: degenerate four-wave mixing (DFWM) and cavity ringdown laser absorption spectroscopy (CRLAS). These two methods can be considered complementary due to different spatial, temporal, and minimum detection sensitivities.;The development phase of this research program focused primarily on DFWM and stimulated Raman scattering (SRS). A tilted backward pump DFWM (t-DFWM) geometry has been shown to improve the signal-to-noise ratio by an order-of-magnitude. It was found that a small tilting angle (ca. ;The application phase of this research program included the detection of CN via the B-X violet band by t-DFWM (the first such measurement reported) in an atmospheric-pressure flame. The transition saturation conditions were determined and allowed for subsequent DFWM CN thermometry measurements.;To determine the feasibility of polyatomic spectroscopy in flames, the DFWM and CRLAS absorption intensities for HCO, CH;The cavity ringdown technique was then successfully implemented to detect HCO via the A-X visible band and CH...