Title: Nonsequential double ionization of helium in IR+XUV two-color laser fields II: Collision-excitation ionization process,
Abstract: The collision-ionization mechanism of nonsequential double ionization (NSDI)
process in IR+XUV two-color laser fields [\PRA \textbf{93}, 043417 (2016)] has
been investigated by us recently. Here we extend this work to study the
collision-excitation-ionization (CEI) mechanism of 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 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 contributions 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 recollsion electron, hence the forward collision plays a dominant role
on 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.