Ionization of hydrogen atom by n-cycle circularly polarized laser pulses

The energy-dependent phenomena of the photoelectron angular distributions (PADs) for different above-threshold ionization (ATI) peaks are investigated by means of the ionization of H atom in few-pulse laser fields with different optical cycle numbers. We find that for one-cycle laser pulse, the PADs and the photoelectron energy spectra are inversion asymmetric, and the emitting direction is not dependent on the photoelectric energy, but only on the carrier-envelope (CE) phase of the laser pulse. For three-cycle laser pulse, although the photoelectron energy spectra are still inversion asymmetric, the emitting direction is dependent on the photoelectric energy, besides the CE phase. This energy-dependent phenomena becomes more obvious for six-cycle laser pulse. When the number of the optical cycle is twelve, however, the photoelectron energy spectra are almost inversion symmetric especially for the low-order ATI peaks, and the bright rings in the momentum distributions are nearly perfect and isotropic. The phenomena show that if the number of cycles of the laser pulse are larger, the ionization behaviors of atom will extend to that of atom in an infinitely monochromatic plane wave.