Recoil collisions as a portal to field assisted ionization at near-UV frequencies in the Strong Field Double Ionization of Helium

Abstract

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 "finger-like" structure for high momenta recently found for $\omega=0.055$ a.u. \cite{Staudte, Rudenko} persists for higher frequencies. At the same time, at $\omega=0.187$ a.u. a new X-shape 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 non-sequential double ionization is discussed.