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Based on the frequency-domain theory, we investigate the above-thresholdionization (ATI) process of an atom in a two-color laser field with infrared(IR) and extreme ultraviolet (XUV) frequencies, where the photon energy of theXUV laser is close to or larger than the atomic ionization threshold. By usingthe channel analysis, we find that the two laser fields play different roles inan ionization process, where the XUV laser determines the ionizationprobability by the photon number that the atom absorbs from it, while the IRlaser accelerates the ionized electron and hence widens the electron kineticenergy spectrum. As a result, the ATI spectrum presents a terrace-likestructure. By using the saddle-point approximation, we obtain a classicalformula which can predict the cutoff of each plateau in the terrace-like ATIspectrum. Furthermore, we find that the difference of the heights between twoneighboring plateaus in the terrace-like structure of the ATI spectrumincreases as the frequency of the XUV laser increases.

Terrace-like structure in the above-threshold ionization spectrum of an atom in an IR+XUV two-color laser field