Role of molecular orbital symmetry on the alignment dependence of high-order harmonic generation with molecules

It is shown that the alignment dependence of the yields of high-order harmonic generation (HHG) from molecules by intense lasers is governed by the orbital symmetry of the outermost electron(s). For N₂, with its outermost σ_g electron, the HHG yield is maximal when the molecules are aligned with the laser polarization direction, in agreement with the recent experiment of Itatani et al. Nature 432 867 (2004)]. For O₂, with its outermost π_g electron, the HHG yield peaks when the molecules are aligned at about 45° from the polarization axis. We emphasize that the alignment dependence is determined mostly by the orbital symmetry and weakly on the laser parameters or the species.