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

In this paper we investigate the ionization of O-2 molecule exposed to few-cycle pulses with different optical cycle numbers by calculating the photoelectron angular distributions (PADs) of O-2 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 O-2 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 02 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.