Background-free cars studies of carbon monoxide in a flame

Background-free coherent anti-Stokes Raman spectra (CARS) of carbon monoxide have been obtained in a rich methane-air flat flame. Use of this background rejection method, requiring only two laser frequencies, results in an increase in signal-to-noise of over 200 in the |x⁽³⁾|² spectrum in comparison with normal CARS. In addition, this technique provides a means for determining the symmetry of observed Raman transitions. The principles of background-free CARS are derived, and the capabilities are demonstrated with experimental data.