  We investigate the orientation dependence of high-order harmonic generation
(HHG) from H$_2^+$ with different internuclear distances irradiated by intense
laser fields both numerically and analytically. The calculated molecular HHG
spectra are found to be sensitive to molecular axis orientation relative to
incident laser field polarization and internuclear separation. In particular,
the spectra calculated for different orientation angles demonstrate a kind of
intersection, which is identified as arising due to intramolecular two-center
interference in the HHG. The striking "intersection" phenomenon can be used to
probe the molecular instantaneous structure.
