Recollision dynamics of electron wave packets in high-order harmonic generation

We numerically investigate the dynamics of recollision of an electron in high-order harmonic generation (HHG) for an H atom and a molecular ion H-2+ using a short (ten optical cycles), and intense (I-0 >= 10(14) W/cm(2)), z-polarized linear laser pulse with wavelength 800 nm by accurately solving the three-dimensional time-dependent Schroumldinger equation. A time-frequency analysis obtained via Gabor transforms is employed to identify electron recollision and recombination times responsible for the generation of harmonics. We find that the HHG spectra are mainly attributed to the recollision of an inner electron wave packet with the parent ion in agreement with the classical recollision model. A time delay of the electron recollision occurs between wave packets in inner and outer regions, near to and far from the parent ion, due to different phase of the acceleration (as well as dipole velocity) of the electron. Inner wave packets at recollision contain mainly short and long trajectories whereas outer wave packets contain only single trajectories. Lower-order harmonics are generated mainly by single recollisions near field extrema, i.e., in strong electric fields whereas higher-order harmonics are generated by double trajectories with different intensities. In the case of H-2+ at a critical nuclear distance for charge resonance enhanced ionization, we also find that HHG mainly comes from contributions of the inner electron wave packet, but with more complex recollision trajectories due to the presence of more than one Coulomb center. Triple recollision trajectories are shown to occur generally for the latter.