Reading molecular messages from high-order harmonic spectra at different orientation angles.

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 the molecular axis orientation relative to incident laser field polarization and internuclear separation. In particular, our simulations demonstrate that at certain harmonic orders the envelopes of the HHG spectra taken at different orientation angles intersect. Moreover, the position of intersection is largely independent of the laser intensity while strongly dependent on the internuclear separation. This striking "intersection" phenomenon is identified as arising due to intramolecular two-center interference in the HHG and can be used to probe the molecular instantaneous structure.