Controlling continuum harmonic generation with attosecond pulses

Photo-ionization and high-order harmonic generation (HOHG) in molecular systems under the combined action of infrared femtosecond (fs) and UV attosecond (as) intense laser pulses are investigated numerically for a one-dimensional non-Born-Oppenheimer H-2(+) ion. It is shown that such a coherent control scheme of fs-as ionization leads to significant enhancement of HOHG, with efficiencies exceeding the one obtained from a single fs pulse by several orders of magnitude. The basic mechanism for this enhancement is established as enhanced ionization and control of the recollision of a high-energy non-zero-velocity ionized electron, as compared to tunneling ionization models where the ionized electron has initial zero velocity. The combined superposition of fs-as pulses results in broad HOHG spectra with a large continuum near the cutoff, thus providing a new source for synthesizing new X-ray (vacuum ultraviolet) as pulses.