A SYSTEMATIC STUDY OF AC STARK SHIFTS IN XENON AT SUPERHIGH LASER INTENSITIES

Experimentally we determine the energy levels of the 4f and 5f states in xenon as a function of laser intensity. In short-pulse (400 fs) pho toelectron spectra we observe an enhancement of the seven-photon ioniz ation yield due to resonances at the six-photon level. Two prominent p eaks are attributed to the 4f and 5f states. Since the peaks are obser ved over a large range of wavelengths (555 nm to 650 nm), we can follo w the energy levels of these states from low to high intensity. The AC Stark shifts of these states are well described by a shift equal to t he ponderomotive shift, up to the highest intensities studied (7 x 10( 13) W cm-2).