Nonsequential double ionization of the hydrogen molecule: Dependence on molecular alignment

We present and analyze results of ab initio computations of nonsequential double ionization of the hydrogen molecule in a few-cycle intense laser pulse. To this end, we have used a model of planar molecules with static nuclei, in which the electronic center-of-mass motion is restricted along the polarization direction of the linearly polarized laser. Alignment of the internuclear axis parallel and perpendicular to the polarization axis of the laser field are considered. We show how the relative electron dynamics, parallel and transversal to the field polarization axis, can be used to resolve the different pathways to nonsequential double ionization. Besides the well-known mechanisms, namely, simultaneous emission of an electron pair upon rescattering and recollisioninduced excitation with subsequent field ionization, we identify two additional pathways. According to our interpretation the latter are due to a decay of doubly excited states, created during the recollision event, and correlated electron emission after soft recollision. Finally, we analyze the relative strengths of the different contributions to nonsequential double ionization as a function of the laser intensity.