Forward degenerate four-wave-mixing spectra of NO in the strong-field regime including polarization, line coupling, and multipole effects. I. Theory - art. no. 012716

In the present paper, we investigate the saturation mechanism of FDFWM (for ward degenerate four-wave-mixing) spectra, observed with different polariza tions of the input beams, line coupling, and Doppler effect. Our theoretica l approach uses a nonperturbative resolution of the density matrix equation s, decomposed on the irreducible tensor basis. This formalism greatly simpl ifies analytical expressions and allows for a correct treatment of magnetic sublevels structure in a three-J level system. Systematic calculations of the FDFWM signal for specific NO lines are performed to characterize intens ities and line shapes as a function of saturation. Relative intensities exh ibit strong changes in the intermediate regime of saturation further suppor ting the necessity for using such a complete model. Under saturating condit ions, line coupling is shown to distort the spectra, according to the type of transition and the polarization configuration. This effect is observed i n the following paper [V. Kruger et al., paper II, Phys. Rev. A 64, 012717 (2001)], where saturated experimental spectra of NO are presented and inter preted for two polarization configurations of interest for diagnostic purpo ses.