  The direct above-threshold ionization (ATI) of diatomic molecules in
linearly-polarized infrared and extreme ultraviolet (IR+XUV) laser fields is
investigated by the frequency-domain theory based on the nonperturbative
quantum electrodynamics. The destructive interference fringes on the
angle-resolved ATI spectra, which are closely related to the molecular
structure, can be well fitted by a simple predictive formula for any alignment
of the molecular axis. By comparing the direct ATI spectra for monochromatic
and two-color laser fields, we found that the XUV laser field can both raise
the ionization probability and the kinetic energy of the ionized electron,
while the IR laser field can broaden the energy distribution of the ionized
electron. Our results demonstrate that, by using IR+XUV two-color laser fields,
the angle-resolved spectra of the direct ATI can image the structural
information of molecules without considering the recollision process of the
ionized electron.
