Verification of the ponderomotive approximation for the ac Stark shift in Xe Rydberg levels

Abstract
We report direct measurements of the ac Stark shifts of Xe Rydberg levels in high-intensity laser fields (up to 6 GW/cm2), demonstrating the expected near-ponderomotive behavior. These measurements provide extensive unambiguous evidence for the validity of perturbative calculations of the nonresonant ac Stark shift at these intensities.