Quantitative analysis of three-photon resonant third-harmonic generation in a nitrogen free-jet zone of silence

International audienceA quantitative analysis of the three-photon resonant third-harmonic generation (THG) obtained in a zone of silence of a supersonic free jet of N/sub 2/ is reported. In this absorbing molecular medium the resonant THG spectra exhibit unusual features. A theoretical model taking into account the fast and large variations of the nonlinear susceptibilities chi/sup (3)/, the wave-vector mismatch DeltaK, and the absorption coefficients alpha/sup (3)/ near the three-photon resonance frequency is developed in order to calculate the harmonic power frequency dependence. It is shown that the harmonic power is sensitive to the phase-matching conditions, leading either to enhancement or depletion of the harmonic signal. The measured extreme-ultraviolet three-photon resonant THG spectra obtained with the b/sup 1/Pi/sub u/(nu'=6) state and recorded for different laser-nozzle distances are found to be in good agreement with the simulated spectr