  Using a three-dimensional semiclassical method, we theoretically investigate
frustrated double ionization (FDI) of Ar atoms subjected to strong laser
fields. The double-hump photoelectron momentum distribution generated from FDI
observed in a recent experiment [S. Larimian et al., Phys. Rev. Research 2,
013021 (2020)] is reproduced by our simulation. We confirm that the observed
spectrum is due to recollision. The laser intensity dependence of FDI is
investigated. We reveal that the doubly excited states of Ar atoms and excited
states of Ar+ are the dominant pathways for producing FDI at relatively low and
high intensities, respectively. Our work demonstrates that at modest
intensities, FDI is a general strong-field physical process accompanied with
nonsequential double ionization and it is an important consequence of
recollision.
