Abstract
We briefly review the theory of harmonic generation. The harmonic distributions with and without the incoherent part are discussed. The time-dependent Schrodinger equation (TDSE) for He with a model potential is solved on a numerical grid. The effects of intermediate-state resonance and of the ionization of He upon the harmonic generation are investigated. The photoelectron spectrum under resonant conditions is obtained in order to verify our arguments of resonant harmonic generation. We also evaluate the contribution of He+ when the laser intensity is sufficiently high for most of the atoms to be ionized during the laser pulse. We find that the harmonics with order higher than 13 are due to the ion when the photon energy is 5 eV, while for photon energy 2 eV the atom produces up to about the 49th harmonic.
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