Attosecond control of ionization and high-order harmonic generation in molecules

Photoionization and high-order harmonic generation (HHG) in molecular systems under the action of combined infrared femtosecond (fsec) and ultraviolet attosecond (asec) laser pulses are investigated numerically for a one-dimensional non-Born-Oppenheimer H₂⁺ ion. It is shown that punctual ``turn-on'' of an asec pulse can trigger the ionization process, which in turn leads to significant enhancement of HHG. The conversion efficiency of HHG can exceed the one obtained from a single fsec pulse by several orders and can be as high as 10^-2 by controlling the recolliding electron responsible for the maximum order in HHG. The HHG spectrum becomes broader and quasicontinuous near the cutoff. This spectrum can be used to synthesize new single x-ray ultraviolet pulses as short as 250 asec.