Abstract
Two-color photoionization processes in rare gases have been studied using the combination of XUV pulses from the Free Electron Laser in Hamburg (FLASH) and intense femtosecond pulses from an external synchronized near infrared laser. In the low field regime of the NIR dressing laser (<1011 W/cm2), the partial cross sections of the two-photon ionization were determined by measuring the polarization dependence of the Above Threshold Ionization (ATI). In the high field regime (>1111 W/cm2), multi-photon processes are dominant and theoretical descriptions beyond time-dependent second order perturbation formalism have to be applied.
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