Nonsequential double ionization of helium in IR plus XUV two-color laser fields II: collision-excitation ionization process

We recently investigated the collision-ionization mechanism of the nonsequential doubleionization (NSDI) process in IR+XUV two-color laser fields [(2016) Phys. Rev. A 93 043417]. Here, we extend this work to study the collision-excitation-ionization (CEI) mechanism of the NSDI processes in the two-color laser fields with different laser conditions. It is found that the CEI mechanism makes a dominant contribution to the NSDI as the extreme ultraviolet (XUV) photon energy is smaller than the ionization threshold of the He+ ion, and the momentum spectrum shows complex interference patterns and symmetrical structures. By channel analysis, we find that, as the energy carried by the recollision electron is not enough to excite the bound electron, the bound electron will absorb XUV photons during their collision; as a result, both forward and backward collisions make a comparable contribution to the NSDI processes. However, it is found that, as the energy carried by the recollision electron is large enough to excite the bound electron, the bound electron does not absorb any XUV photon and it is excited only by sharing the energy carried by the recollision electron, hence the forward collision plays a dominant role in the NSDI processes. Moreover, we find that the interference patterns of the NSDI spectra can be reconstructed by the spectra of two above-threshold ionization (ATI) processes, which may be used to analyze the structure of the two separate ATI spectra by NSDI processes.