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) photoelectron spectra we observe an enhancement of the seven-photon ionization yield due to resonances at the six-photon level. Two prominent peaks are attributed to the 4f and 5f states. Since the peaks are observed over a large range of wavelengths (555 nm to 650 nm), we can follow 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 the ponderomotive shift, up to the highest intensities studied (7 x 10(13) W cm-2).