Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation

The theoretical developments presented in a preceding companion paper by Wassaf et al. [Phys. Rev. A 67, 053405 (2003)], for simulating photoelectron spectra, are used to address several issues regarding the harmonic generation process. Both above-threshold Ionization (ATI) and high-order harmonic generation are observed when atoms are submitted to a laser field with intensity around I=10¹⁴W cm^-2. Here, we demonstrate that the resonances, together with multiple recollisions processes, which have been shown to be at the origin of enhancements of the magnitudes of ATI peaks in the high-energy range, can also play a determining role on the magnitudes of harmonic lines within the plateau. These findings have been obtained via a set of quantum and classical simulations for two classes of one-dimensional model potentials, i.e., either long range (Coulomb-like) or short range (with an exponentially decreasing tail). They are confirmed by following the time evolution of the emission rate of selected harmonics with the help of a (waveletlike) Gabor time-frequency analysis.