Reading molecular messages from the intersections of high-order harmonic spectra at different orientations

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.