Laser induced light shift in helium: Theory and experiment

Peter Schlagheck, P. Lambropoulos

Thomas Halfmann, Леонід Петрович Яценко, M. Shapiro +2 more

We report the observation of laser-induced structure (LICS) in the flat photoionization continuum of helium. The structure is a strong and spectrally sharp resonance, showing both enhanced and diminished ionization (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 also show the effect of dynamic Stark shifts upon the LICS profile. A discussed theoretical model, incorporating significant dynamic Stark shifts, gives a quantitatively accurate description of the line profile for a wide range of pulse intensities.

Alexander Magunia

Within this work, electron dynamics of autoionizing, doubly excited states in helium are investigated. Therefore, strong field couplings of these states are studied in two spectral regimes: in the infrared and in the extreme ultraviolet. A laser system capable of tuning its output-pulses' central wavelength within the infrared spectral regime is installed and characterized by a FROG setup. Furthermore, high harmonics of these output pulses are generated and discussed. First intensity dependent and time resolved experiments in helium with this laser system are presented and supported by numerical calculations. The transition from an unperturbed light induced state, which is generated from the 2p4p state, to a resonant Autler-Townes splitting of the sp2;3+ state is observed. Furthermore, strong field couplings between the ground and a doubly-excited state are studied in the extreme ultraviolet only regime. With the help of simulations Rabi-like couplings and Stark shifts for ultrashort pulses leading to a manipulated absorption line shape are investigated. The same study is carried out for autoionizing states. This theory part is used to explain experimentally measured line-shape changes of the 2s2p resonance in helium interacting with FEL pulses.

T. Yabuzaki, Toshiya Kinoshita, K. Fukuda +1 more

R. Keil, P. E. Toschek

Light-induced resonance frequency shifts of atoms and molecules under off-resonance conditions are considered. The results are reported of the application of a cross-saturation method to shifts inside a Doppler profile of an atomic helium line. Two-photon Lamb dips are reported and these require perturbation theory calculations up to the fifth approximation.

Ti Chuang