  Based upon our previous works (Eur.Phys.J.D 6, 319(1999); Chin.Phys.Lett. 18,
236(2001)), we develop a classical approach to calculate the high-order
harmonic generation of the laser driven atoms and molecules. The Coulomb
singularities in the system have been removed by a regularization procedure.
Action-angle variables have been used to generate the initial microcanonical
distribution which satisfies the inversion symmetry of the system. The
numerical simulation show, within a proper laser intensity, a harmonic plateau
with only odd harmonics appears. At higher intensities, the spectra become
noisier because of the existence of chaos. With further increase in laser
intensity, ionization takes place, and the high-order harmonics disappear. Thus
chaos introduces noise in the spectra, and ionization suppresses the harmonic
generation, with the onset of the ionization follows the onset of chaos.

Atomic Physics