Observation of ac Stark-effect-enhanced four-wave mixing in a near-resonant four-level system

Two laser beams at frequencies ν₁ and ν₂ are used to produce nonlinear scattering (wave mixing) in I₂ vapor at frequency 2ν₁+ν₂. The scattering results from near-resonant allowed transitions in I₂ molecules and is observed to be anomalously strong when both laser beam frequencies are detuned in the same direction from frequencies corresponding to resonance in the free molecule. The strong scattering is interpreted in terms of a resonantly enhanced ac Stark shift caused by a configuration interaction between the ``dressed molecular levels.'' By tuning the two pulses simultaneously, a tuning range of approximately 2 A{} (50 cm^-1) is achieved in the mixed wave. The scattering process should therefore have practical application in the efficient generation of tunable coherent radiation in the ultraviolet and vacuum-ultraviolet range of the spectrum. Some of the spectroscopic properties of the three-quantum state at E{~}53 000 cm^-1 are studied.