Influence of the alignment angle of molecules on the cutoff of the high-order harmonics

Abstract

Using the Lewenstein model combined with the Ammosov–Delone–Krainov (ADK) ionization theories, the high-order harmonic generation (HHG) of the diatomic molecules O2 and N2 at the various alignment angles are investigated. We find that the cutoff of harmonic spectra from the aligned O2 and N2 are influenced by the alignment angles. The dependence of the cutoff energy on the alignment angle is attributed to the anisotropy in the molecular ionization, which is rooted in the nonuniform electron distributions of the highest occupied molecular orbital. If the polarization of the laser crosses the denser region of the electron cloud, the ionization rate is larger, and then the saturation laser intensity is smaller, therefore the harmonic spectra are cut off at the lower energy.