Mechanisms of ultrahigh-order harmonic generation

We analyze mechanisms of high-order harmonic generation from two-center molecules at large internuclear separations in linearly polarized laser fields. Since laser-driven electrons acquire instantaneous kinetic energies of up to 8 times the ponderomotive potential Up, recombination at an appropriately placed nucleus leads to ultrahigh-order harmonic emission [P. Moreno, L. Plaja, and L. Roso, Phys. Rev. A 55, R1593 (1997)]. By solving the time-dependent Schrödinger equation for model systems, we show that this mechanism is only efficient if the single-particle orbital of the electron is coherently delocalized over the two potential wells. This is realized in the ground state of a one-electron molecular ion. In a neutral molecule or in a molecular ion created by ionization of a neutral molecule at large internuclear distance, the coherence is destroyed by electron-electron correlation