Two-photon resonance spectroscopy of vibrationai transitions in molecules under four-wave frequency mixing conditions

Generation of the sum and difference frequencies ω_vis±2ω_ir as a result of coherent four-wave frequency mixing of quasiresonance (relative to vibrationai transitions) CO₂ laser radiation and of visible radiation in the form of the second harmonic of a YAG:Nd laser was studied in media consisting of various molecules (SF₆, BCl₃, CF₃Br, CF₃I). The signal was very sensitive to tuning of the infrared radiation frequency relative to the Q branch of a two-photon vibrationai transition. A theoretical model was developed which ensured a good agreement with the experimental dependence of the intensity of the signal on the infrared radiation frequency in the case of gaseous CF₃Br. It was predicted theoretically that the investigated process should cause a considerable narrowing of the Q branch of a two-photon transition compared with the usual thermal width.