Saturation and Spectral Line Behavior in the Resonant CARS Spectrum of OH.

Abstract : The origins of the unpredicted satellite lines about the central resonance of the CARS spectrum found in a previous study have been explored. The objectives of the program were to determine the dependence of the line structure on the input laser intensities and on tuning of the the resonant frequency. In the previous AFOSR-supported study, electronically resonant coherent anti-Stokes Raman spectroscopy (CARS) of OH was demonstrated for the first time using a flame source. Theory and experiment were in generally good agreement, except for the presence of satellite lines (extra resonances) about the strong central component. In order to investigate reliably the power dependence of the resonance CARS spectrum and thereby test for saturation effects, a single mode, pulsed, tunable dye laser was utilized to insure selective excitation of a single resonance. The resultant spectra show clearly that saturation occurs at input laser pulse energies of the resonant frequency of about 0.1 millijoules. Saturation effects are particularly apparent when the CARS pump frequency is on line center of the selected electronic transition. Tuning away from line center and/or reducing input pulse energy produces a notable decrease in the strength of the satellite lines relative to the central peak. The frequency splitting between the satellite components was determined to be independent of laser intensity and the tuning of the resonant CARS pump frequency relative to line center.