Thermometry by degenerate four-wave mixing: J dependence of laser-induced grating reflectivity

Abstract

Irreducible tensor operator formalism is applied to the theory of resonant degenerate four-wave mixing spectroscopy, and third-order perturbation theory is used to obtain an expression for the molecular polarisation. This is applied to predict the J dependence of the signal strength expected for Hund's case (a) molecules. Comparison with the corresponding one-photon line-strength factor is made, and the effect on temperature determinations by degenerate four-wave mixing in combustion studies is discussed. For Hund's case (a) molecules in the weak-field limit our results show that degenerate four-wave mixing spectroscopy will lead to a significant underestimate of the flame temperature.