Nonsequential double ionization （NSDI） processes of nonaligned diatomic molecules N2 and O2 are studied using the S-matrix theory. Our results show that the NSDI process significantly depends on the molecular symmetry and structure. The ratio of NSDI rate to single ionization rate as a function of the field intensity is obtained. It is found that N2 behaves closely with its companion atom Ar in the ratios over the entire intensity range, while O2 exhibits an obvious suppression effect, which is qualitatively consistent with the experiment.
We investigate the low-energy structure （LES） in the above-threshold ionization spectrum at a mid-infrared laser wavelength with a semiclassical model. Using a softened Coulomb potential （CP） and changing the softening parameter, we show that though the very low-energy structure （VLES） and high low-energy structure （HLES） are both due to the interaction between the ionic CP and the electron, the two structures have different physical mechanisms： the VLES can be attributed to the electron-ion Coulomb interaction at a rather small distance and the HLES is more likely to be ascribed to the electron-ion Coulomb interaction at a large distance.
