Classical calculation of high-order harmonic generation of atomic and molecular gases in intense laser fields

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 that satisfies the inversion symmetry of the system. The numerical simulation show, with a moderate 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.