  Using the frequency-domain theory, the high-order above threshold ionization
(HATI) of a molecule is treated as a two-step transition: an above threshold
ionization (ATI) followed by a laser-assisted collision (LAC). It is found that
the HATI spectrum carries three pieces of information: the fingerprint of the
molecular wave function lying in the ATI transition, the geometry structure of
the molecule lying in the potential scattering between two plane waves, and the
interaction between the ionized electron and the laser field lying in a phase
factor in the LAC transition, where the phase factor can be regarded as due to
the action difference of the classical trajectory before and after the
collision within one laser cycle. Based on this picture, it is demonstrated
that the skeleton structure of a molecule can be imaged more clearly using a
relatively higher frequency laser field due to the simplification of
interference originated from the phase factor. This work can shed light on
imaging the structure of a complex molecule by a HATI process.
