We theoretically study infrared (IR)-driven high-order harmonic generation (HHG) assisted by attosecond pulses with a central energy above the atomic ionization threshold. We provide a clear physical picture for controlling HHG using the time delay between the attosecond pulses and the IR laser reported by Faria et al. [Phys. Rev. A74, 053416 (2006)]. This physical picture also indicates that the combined attosecond pulses and IR laser can help resolve the dynamics of ionized electrons from time-dependent harmonic spectra. We present the quantum effect on HHG as an example. While leaving parent ions, ionized electrons can still emit harmonics in the semi-classically forbidden situation. The two-color excitation provides a practical method to observe the quantum effect experimentally. Furthermore, in our work, attosecond pulses and an IR field are considered with a realistic pulse shape, which shows a quantitatively important effect in controlling harmonic spectra. Accordingly, a guide to optimize the control capability for HHG is presented, and a method to determine the IR carrier-envelope phase based on the pulse-shape effect on the HHG is also proposed.
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