Rotational line strengths in degenerate four wave mixing

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Abstract
In this paper we present a derivation of the rotational line strengths involved in fully resonant degenerate four wave mixing processes (DFWM). General expressions are given for diatomic molecules with different Hund's coupling cases. More precisely, we tabulated the rotational line strengths for the electronic transition 2Σ+ -2Π of OH and NO. For the ground state, Hund's cases (b), (a) and the intermediate case (a-b) are considered. We also stress the dependence of the line strengths with respect to the geometry and to the polarization of the incident beams. So, we consider the optical phase conjugation (OPC) geometry and the forward (FDFWM) one. We also give expressions for the line strengths in different polarization arrangements, i.e. parallel or crossed polarized beams. A comparison is made between these cases. Finally, theoretical simulations, using our line strengths calculations, are compared with experimental spectra of OH obtained in a methane-air flame using OPC and FDFWM.