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 1013 W cm−2. Already at modest intensities of the NIR dressing field, the XUV-induced photoionization 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 cut-off energies of the sideband plateau are in good agreement with a classical model. The detailed structure of the two-color spectra, including the formation of 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.
Highlights
E0 are the angular frequency and the field amplitude of the NIR pulse
Note that for intermediate cases, where τFXWUHVM ≈ TNIR, electrons are emitted within about one period of the NIR-laser field and the electron spectra are characterized by more complex structures, as discussed in detail by Kazansky et al [13] and observed experimentally recently [14]
By making use of the streaking regime, the arrival time and the temporal profile of the XUV pulses from the Free-electron Laser in Hamburg (FLASH) were measured with a temporal resolution of better than 10 fs [10], when combining the XUV and THz pulses generated in the same linear accelerator
Summary
E0 are the angular frequency and the field amplitude of the NIR pulse. The gray-shaded area indicates the XUV laser intensity envelope, i.e. the temporal rate for electron release. The observed features in the electron spectrum depend strongly on the NIR laser intensity and are sensitive to the temporal and spatial overlap of the two pulses. Therein the sideband-averaged features of the experimental electron spectra are compared to the predictions of the classical theory, whereas purely quantum mechanical features are discussed in comparison to the TDSE results.
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