Laser-induced nonsequential double ionization in diatomic molecules: one- and two-centre rescattering scenarios

We investigate laser-induced nonsequential double ionization from aligned diatomic molecules, using the strong-field approximation in its length and velocity-gauge formulations. Throughout, we consider that the first electron dislodges the second by electron-impact ionization. Employing modified saddle-point equations, we single out the contributions of different scattering scenarios to the maxima and minima observed in the differential electron-momentum distributions. We show that the quantum interference between the electron orbits starting and ending at a specific centre C-j, and those starting at C-j and ending at a different centre C-v, leads to the same maxima and minima as if all possible scenarios are taken. There exist, however, quantitative differences as far as the gauge choice is concerned. Indeed, while the velocity-gauge distributions obtained employing only the above-mentioned processes are practically identical to the overall distributions, their length-gauge counterparts exhibit an asymmetry in the positive and negative momentum regions. This asymmetry is due to additional potential-energy shifts which are only present in the length-gauge formulation, and which, depending on the centre, sink or increase the potential barrier through which the first electron tunnels. In contrast, the interference between topologically similar scenarios leads at most to patterns whose positions, in momentum space, do not agree with the overall interference condition, neither in the length nor in the velocity gauge.