Double ionization of nitrogen molecules in orthogonal two-color femtosecond laser fields

Double ionization of nitrogen molecules in orthogonally polarized two-color femtosecond laser fields is investigated by varying the relative intensity between the fundamental wave (FW) and its second harmonic (SH) components. The yield ratios of the double ionization channels, i.e., the non-dissociative N-2(2+) and Coulomb exploded (N+, N+), to the singly charged N-2(+) channel exhibit distinct dependences on the relative strength between the FW and SH fields. As the intensity ratio of SH to FW increases, the yield ratio of (N+, N+)/N-2(+) gradually increases, while the ratio of N-2(2+) /N-2(+) first descends and then increases constituting a valley shape which is similar to the behavior of Ar-2+Ar-/(+) observed in the same experimental condition. Based on the classical trajectory simulations, we found that the different characteristics of the two doubly ionized channels stem from two mechanisms, i.e., the N-2(2+) is mostly accessed by the (e, 2e) impact ionization while the recollision-induced excitation with subsequent ionization plays an important role in producing the (N+, N+) channel.