Influence of barrier suppression in high-order harmonic generation

High-order harmonic generation accompanying photoionization presents a clear plateau structure in situations where tunneling is the fundamental process for the ejection of the electron from the core. We describe here what is expected to happen when the laser intensity is so high that barrier suppression begins to be the dominant mechanism. We show that this is an ionization regime where harmonic generation decreases with intensity, obtaining therefore an upper practical limit for high harmonic generation. Thus we are able to predict situations where the harmonic cutoff appears at a lower energy than that expected by the standard I_P+3.17U_P law where I_P and U_P correspondingly are the ionization threshold for an electron in the ground state and ponderomotive energy.