Ionization of O2 molecule by n-cycle circularly polarized laser pulses

In this paper we investigate the ionization of O2 molecule exposed to few-cycle pulses with different optical cycle numbers by calculating the photoelectron angular distributions (PADs) of O2 molecule. In order to show the influence of the molecular electronic structure and the laser pulse on the PADs, we also calculate the PADs of H atom, which has an isotropic electronic distribution in its ground state. We find that for one-cycle laser pulse, the PADs of O2 molecule are inversion asymmetric, and the PADs are not dependent on the photoelectron energy. For two-cycle and three-cycle laser pulses, although the PADs are still inversion asymmetric, the PADs have a certain dependence on the photoelectron energy. According to this phenomenon, we can control the emission directions of photoelectrons by choosing a fitted photoelectron energy. At same time, with a fitted photoelectron energy, by changing the carrier-envelope (CE) phase, the split position of the PADs of O2 molecule can be controlled, and then can forecast the main emission direction of photoelectron. While for long pulse (ten-cycle pulse) the dependence of PADs on the photoelectron energy nearly disappears, and the ionization behaviors extend to that in an infinitely monochromatic plane wave.