LASER-INDUCED THIRD HARMONIC GENERATION IN FORBIDDEN REGIONS.

Laser third-harmonic generation in gaseous media has long been established as a practical method for the generation of short-wavelength radiation. A very general theoretical treatment predicts that introduction of a second laser field in near resonance with a one-photon allowed transition between the three-photon level and some higher level should influence the generation of third-harmonic light. Specifically, it predicts third-harmonic generation in normally positively dispersive regions, and, in the same conditions, changes in the third-harmonic yield in the negatively dispersion regions. Modifications of third-harmonic generation by a second strong laser have been studied. Tunable third harmonic light has been observed in the positively dispersive region on the low-energy side of the xenon 6s double prime left bracket 1/2 right bracket **0//1 state induced on introduction of a second strong laser field. Strong systematic increases and decreases in ionization signals generated to the blue of the 6s prime level have also been observed. All the results are interpreted in terms of phase-matching changes induced by the second laser, which affect third-harmonic production and subsequent ionization of xenon atoms.