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The collision-ionization mechanism of nonsequential double ionization (NSDI)process in IR+XUV two-color laser fields [\PRA \textbf{93}, 043417 (2016)] hasbeen investigated by us recently. Here we extend this work to study thecollision-excitation-ionization (CEI) mechanism of NSDI processes in thetwo-color laser fields with different laser conditions. It is found that theCEI mechanism makes a dominant contribution to the NSDI as the XUV photonenergy is smaller than the ionization threshold of the He$^+$ ion, and themomentum spectrum shows complex interference patterns and symmetricalstructures. By channel analysis, we find that, as the energy carried by therecollision electron is not enough to excite the bound electron, the boundelectron will absorb XUV photons during their collision, as a result, bothforward and backward collisions make a comparable contributions to the NSDIprocesses. However, it is found that, as the energy carried by the recollisionelectron is large enough to excite the bound electron, the bound electron doesnot absorb any XUV photon and it is excited only by sharing the energy carriedby the recollsion electron, hence the forward collision plays a dominant roleon the NSDI processes. Moreover, we find that the interference patterns of theNSDI spectra can be reconstructed by the spectra of two above-thresholdionization (ATI) processes, which may be used to analyze the structure of thetwo separate ATI spectra by NSDI processes.

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