The application of Doppler-free two-photon electronic spectroscopy for the in situ monitoring of stratospheric gases has been analyzed. Expressions have been derived for the optimum detection pressure and maximum volumetric sensitivity. The results indicate that detection limits of the order of 105 to 107 molecules/cm3 are feasible for molecular species with strong two-photon uv transitions that undergo efficient radiative relaxation from the excited state. The minimum detectable concentration of trace gas has been plotted vs altitude.
Jerry Gelbwachs, "Doppler-free two-photon spectroscopy for monitoring stratospheric gases: an analysis," Appl. Opt. 15, 2654-2657 (1976)