Laser-induced third-harmonic generation in forbidden regions

New observations are reported of tunable vacuum ultraviolet light generated in the forbidden region to the red of the xenon $6s^{\prime }{\left[\frac{1}{2}\right]}_1^0$ state due to the introduction of a second, resonant laser field. Striking observations of double-resonant ionization signals on the blue side of the xenon $6s^{\prime }$ level are also reported. Both sets of data are interpreted in terms of the changes in phase matching and the nonlinear susceptibility induced in the medium by the second laser. These changes are successfully analyzed in terms of the ac Stark effect in the three-photon level due to the second laser field.