High-order harmonic cutoff extension of the O₂ molecule due to ionization suppression

High-order harmonic generation has been observed experimentally from O₂ molecules at the saturation ionization intensity. The harmonic cutoff extends far beyond the cutoff of Xe despite both have nearly equal ionization potentials. In contrast, the harmonic spectra for N₂ and Ar, which have almost the same ionization potentials, are essentially close to each other. We show the extension of harmonic cutoff in O₂ is a consequence of ionization suppression. Using a simple modified tunneling ionization model for molecules, we predict both the harmonic cutoff extension and the ionization suppression semiquantitatively.