POPULATION TRAPPING AND LASER-INDUCED CONTINUUM STRUCTURE IN HELIUM -EXPERIMENT AND THEORY

We report the observation of laser-induced structure (LICS) in the fla t photoionization continuum of helium. The structure is a strong and s pectrally sharp resonance, showing both enhanced and diminished ioniza tion (a Beutler-Fano profile). Our observations show, in an otherwise unstructured continuum, pronounced ionization suppression (as much as 70%), which is due to population trapping associated with LICS. We als o show the effect of dynamic Stark shifts upon the LICS profile. A dis cussed theoretical model, incorporating significant dynamic Stark shif ts, gives a quantitatively accurate description of the line profile fo r a wide range of pulse intensities.