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Abstract

We present a systematic study of the photoionization of noble gas atoms exposed simultaneously to ultrashort (20 fs) monochromatic (1–2%spectral width) extreme ultraviolet (XUV) radiation from the Free-electron Laser in Hamburg (FLASH) and to intense synchronized near-infrared (NIR) laser pulses with intensities up to about 1013Wcm−2. Already at modest intensities of the NIR dressing field, the XUV-induced hotoionization lines are split into a sequence of peaks due to the emission or absorption of several additional infrared photons. We observed a plateau-shaped envelope of the resulting sequence of sidebands that broadens with increasing intensity of the NIR dressing field. All individual lines of the nonlinear two-color ionization process are Stark-shifted, reflecting the effective intensity of the NIR field. The intensity-dependent cutoff energies of the sideband plateau are in good agreement with a classical model. The detailed structure of the two-color spectra, including the formationof individual sidebands, the Stark shifts and the contributions beyond the classical cut-off, however, requires a fully quantum mechanical description, as is demonstrated with time-dependent quantum calculations in single-active electron approximation.

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