Valence orbital method of calculating harmonic emission from diatomic molecule

We present a valence orbital method of calculating high-order harmonic generation from a diatomic molecule with arbitrary orientation by using a space rotation operator. We evaluate the effects of each valence orbital on harmonic emissions from N2 and O2 molecules in detail separately. The calculation results confirm the different properties of harmonic yields from N2 and O2 molecules which are well consistent with available experimental data. We observe that due to the orientation dependence of σ and π orbitals, the bonding orbital (σ2pz)2 of N2 determines the maximum of harmonic emission when the molecular axis of N2 is aligned parallel to the laser vector, and the magnitude of the high harmonic signal gradually weakens with the orientation angle of molecular axis increasing. But for O2 molecule the antibonding orbitals (π*2py)1 and (π*2pz)1 contribute to the maximum of harmonic yield when O2 is aligned at 45° and bonding orbitals (π*2py)2 and (π*2pz)2 slightly influence the orientation angle of maximum of harmonic radiation not exactly at 45°. (atomic and molecular physics)