Comparison of high-order harmonic generation of Ar atoms and H$_{2}$ molecules in intense 780~nm laser fields

We analyze the high-order harmonic generation (HHG) of Ar atoms and H$_{2}$ molecules in 780~nm laser fields with the pulse duration about 30~fs and various peak intensities by means of the self-interaction-free time-dependent density functional theory (TDDFT). Since the ionization potentials of Ar and H$_{2}$ are close to each other, the cutoff position of the HHG spectra for the specific intensity is expected approximately at the same harmonic order, according to the three-step model. In general, our TDDFT calculations agree with this prediction; however, in the high-energy part of the HHG spectra, the harmonic signal from H$_{2}$ is considerably lower than that from Ar. On the other hand, the HHG spectrum of Ar has a prominent minimum at the photon energy 50~eV, especially for lower laser intensities. This minimum has the same nature as the well-known Cooper minimum in Ar observed in photoionization cross sections.