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 sHHGd from molecules by intense lasers is governed by the orbital symmetry of the outermost electronssd. For N2, with its outermost sg 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. fNature 432, 867 s2004dg. For O2, with its outermost pg 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.