Resonant CARS Detection of OH Radicals.

Abstract : Under this AFOSR contract, resonant CARS (Coherent Anti-Stokes Raman Spectroscopy) of OH has been observed for the first time. Resonant CARS was achieved for several electronic transitions in the ultraviolet A--X bands of OH in a methane/oxygen flame. Saturation of the resonant CARS spectrum of OH was examined experimentally and definite evidence found, both in the intensity variation, and the number and shapes of the lines. Concurrently with the experimental studies, a good understanding of the theoretical aspects of the resonant CARS process has been secured, to the extent that predicted resonant CARS spectra can be computer synthesized and graphed for any selected frequency. A major departure from theoretical predictions was the experimental observation of satellite lines about the central line, not predicted from theory. At present, the cause of these extra resonances has not been determined, but their appearance may arise from saturation effects, or be caused by an undetermined nonlinear optical effect (such as dephasing--nduced coherences). Another contributing factor may be rotational energy level transfer bought about by collision processes, occurring within the duration of the 10 nanosecond laser phase.