This paper studies the molecular rotational excitation and field-free spatial alignment in a nonresonant intense laser field numerically and analytically by using the time-dependent Schrdinger equation.The broad rotational wave packets excited by the femtosecond pulse are defined in conjugate angle space,and their coefficients are obtained by solving a set of coupled linear equations.Both single molecule orientation angles and an ensemble of O2 and CO molecule angular distributions are calculated in detail.The numerical results show that,for single molecule highest occupied molecular orbital(HOMO) symmetry σ tends to have a molecular orientation along the laser polarization direction and the permanent dipole moment diminishes the mean of the orientation angles;for an ensemble of molecules,angular distributions provide more complex and additional information at times where there are no revivals in the single molecule plot.In particular,at the revival peak instant,with the increase of temperature of the molecular ensemble,the anisotropic angular distributions with respect to the laser polarization direction of the πg orbital gradually transform to the symmetrical distributions regarding the laser polarization vector and for two HOMO configurations angular distributions of all directions are confined within a smaller angle when the temperature of the molecular ensemble is higher.
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 (π2py)2 and (π2pz)2 slightly influence the orientation angle of maximum of harmonic radiation not exactly at 45 .
Using a neutral N2 beam as target,this paper studies the dissociation of N2+ in intense femtosecond laser fields(45 fs,～1×10 16 W/cm 2) at the laser wavelength of 800 nm based on the time-of-flight mass spectra of N + fragment ions.The angular distributions of N+ and the laser power dependence of N + yielded from different dissociation pathways show that the dissociation mechanisms mainly proceed through the couplings between the metastable states(A,B and C) and the upper excited states of N2+.A coupling model of light-dressed potential energy curves of N+2 is used to interpret the kinetic energy release of N+.
