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.