  High-order harmonic generation (HHG) is investigated theoretically in the
over-the-barrier ionization (OBI) regime revealing the strong signature of
interference between two separately ionized and separately propagating free
wave packets of a single electron. The interference leads to the emission of
coherent light at a photon energy corresponding to the kinetic-energy
difference of the two recolliding electron quantum paths, thus complementary to
the well-known classical three-step picture of HHG. As will be shown by
time-frequency analysis of the emitted radiation, the process entirely
dominates the coherent HHG emission after the atomic ground state has been
depleted by a strong field. Moreover, it can be isolated from the
continuum--bound harmonics via phase-matching.
