Low-momentum anticorrelated electron emission in mid-infrared double ionization of argon

We investigate wavelength-dependent nonsequential double ionization of Ar from the near- to mid-infrared regime using a full-Coulomb three-dimensional semiclassical ensemble model. As the wavelength increases from 800 nm and 1300 nm to 3100 nm, the correlated two-electron momentum distribution evolves from a fingerlike pattern to a crosslike pattern and develops low-momentum anticorrelated electron emission in quadrants II and IV. Trajectory analysis shows that the fingerlike structures are associated with Coulomb-assisted large-angle backscattering, whereas the mid-infrared spectrum gives larger weight to forward-scattering-like recollision trajectories and small longitudinal momenta. The low-momentum anticorrelated component does not arise from prompt impact ionization at recollision. Instead, recollision prepares a correlated or excited near-core configuration that completes double escape in a later field window near an electric-field maximum, where the vector-potential contribution to the longitudinal drift is small. These results show that high-U_\rm p anticorrelated electron emission is shaped jointly by recollision energy transfer and by the field phase at final escape.