High-order harmonic cutoff extension of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>molecule due to ionization suppression

High-order harmonic generation has been observed experimentally from ${\mathrm{O}}_{2}$ 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 ${\mathrm{N}}_{2}$ and Ar, which have almost the same ionization potentials, are essentially close to each other. We show the extension of harmonic cutoff in ${\mathrm{O}}_{2}$ 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.

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