  The intensity-dependent resonance-like enhancement phenomenon in high-order
above-threshold ionization spectrum is a typical quantum effect for atoms or
molecules in the intense laser field, which has not been well understood. The
calculations of TDSE are in remarkable agreement with the experimental data,
but they can not clarify the contributions of the bound states. The
semi-classical approach of strong field approximation, in which no excited
states are involved, can obtain the similar phenomenon, but the laser
intensities of enhanced regions predicted by SFA are inconsistent with the
results of TDSE. In this letter, a new fully quantum model is established from
TDSE with not any excited states. Two types of enhanced structures, unimodal
and multimodal structures, are found in the results of TDSE and model. Besides,
the calculations of model reproduce the key features of the results of TDSE. It
shows that the excited states are not the key factor in the resonance-like
enhancements in our calculated system, since there are not any excited states
in our model. Based on the calculations of our model, we show that such
resonance-like enhancements are caused by the constructive interference of
different momentum transfer channels. Last, the Fano-like lineshapes are also
discussed for the features of multi-channel interference.
