We experimentally investigate the atomic ionization process in an extreme-ultraviolet (XUV) pulse train generated by the high-order harmonic generation process and an IR laser field. Compared with the electron spectrum ionized only by the XUV pulse train, the electron energy spectra generated in the two-color field exhibit two characterizations: (1) the spectrum is smoothed in the presence of a weak IR field, and (2) the spectrum is asymmetrically broadened when using an intense IR field. Simulation results based on the frequency-domain theory qualitatively agree with the experimental spectra. We demonstrate that the asymmetrically broadened spectrum could be interpreted by a clear two-step ionization picture: an electron wavepacket is ionized from the atomic ground state by the XUV pulse train, and then the energy of it is asymmetrically changed by absorbing or emitting specific IR photons.
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