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In this paper we analyse the effect of coherent saturation in the laser-induced fluorescence (LIF) of molecules. We develop an earlier model introduced by Billy et al. in 1987 of this coherent effect and propose a simple formula predicting the line shape and the line intensity for a laser-molecule interaction time considerably longer than the excited-state radiative lifetime. The predictions of this formula are tested on a LIF experiment in which a Na2 molecular beam is excited with a continuous-wave single-frequency dye laser. Considerable spectral broadening, up to two hundred times the natural linewidth is observed, in good agreement with the model.

The technique of laser induced detection of chemical reaction products has been used to study the reaction: I2 + F → IF + I. Using a single frequency dye laser and an accurate wavenumber measurement, the X 1Σ+ → B 3Π(0+) excitation spectrum of the product IF molecules has been studied with high resolution over a large spectral range (700–789 nm and 837–873 nm). Due to the exoenergicity of the reaction very high rovibrational levels are populated in the X state up to ν″ = 20. Our measurements, together with those of previous authors, have allowed us to obtain an improved and extended spectroscopic representation of the X and B states of IF in terms of Dunham coefficients.

Laser induced fluorescence detection of reaction products is now a wide-spread technique. When the laser coherence time is longer than the time spent by a molecule in the laser beam, the saturation effects of the laser-molecule interaction cannot be described by rate equations. In this paper we present a simple theoretical description of these effects and detailed experimental verifications. These results are quite important if one wants to derive reliable population measurements from the laser induced fluorescence signals in the case of coherent saturation.