THERMOMETRY BY DEGENERATE 4-WAVE-MIXING - J DEPENDENCE OF LASER-INDUCED GRATING REFLECTIVITY

Irreducible tensor operator formalism is applied to the theory of reso nant degenerate four-wave mixing spectroscopy, and third-order perturb ation theory is used to obtain an expression for the molecular polaris ation. This is applied to predict the J dependence of the signal stren gth expected for Hund's case (a) molecules. Comparison with the corres ponding one-photon line-strength factor is made, and the effect on tem perature determinations by degenerate four-wave mixing in combustion s tudies is discussed. For Hund's case (a) molecules in the weak-field l imit our results show that degenerate four-wave mixing spectroscopy wi ll lead to a significant underestimate of the flame temperature.