No abstract

No abstract

This paper presents a study of OH detection by degenerate four-wave mixing (DFWM) in a premixed laminar CH4-air flame, at pressures varying from 1 to 7 bar. The P and Q branches of the A 2 Sigma +-X 2 Pi electronic transition are observed. A spectroscopic study is performed and saturation and pressure effects on the lineshapes and concentration profiles are presented. A preliminary comparison between CARS and DFWM is given.

The present paper deals with a three-level model of interaction with three crosspolarized and co-propagating fields having the same frequency. The two pump waves have arbitrary intensities and the third one (the probe wave) is assumed to be non-saturating. Using the radiative renormalization method, density matrix equations are solved analytically. The forward degenerate four-wave mixing (FDFWM) signal is calculated under thermodynamic conditions where collisional and Doppler broadenings are comparable. The calculation also takes into account crossed-polarization effects. The FDFWM intensity is computed as a function of power densities and pressure. FDFWM experimental spectra of the 2 + -2 (0-0) band of OH obtained in a welding torch flame are compared to theoretical profiles. For incident laser intensities in the range of a few MW cm −2 , a good agreement is obtained between calculated and experimental profiles. Discrepancies arise under stronger conditions of saturation, showing the limits of application of our model. However, because there is little virtue in using higher laser powers, the model should be extremely useful in practical applications. In particular, we demonstrate that the signal intensity is optimal and the pressure dependence is minimized when the saturation parameter is unity.