Recoil collisions as a portal to field-assisted ionization at near-uv frequencies in the strong-field double ionization of helium

We explore the dependence of the double ionization of the He atom on the frequency of a strong laser field while keeping the ponderomotive energy constant. As we increase the frequency we find that the remarkable fingerlike structure for high momenta recently found for omega=0.055 a.u. persists for higher frequencies. At the same time, at omega=0.187 a.u., a new X-shaped structure emerges for small momenta that prevails in the correlated momenta distribution. The role of this structure as a signature of the frequency dependence of nonsequential double ionization is discussed.