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

TL;DR

This study investigates how the frequency of a strong laser field influences double ionization of helium, revealing persistent and new momentum structures that shed light on the underlying ionization mechanisms.

Contribution

It demonstrates the frequency dependence of ionization structures in helium, highlighting the persistence of finger-like features and the emergence of an X-shape pattern at higher frequencies.

Findings

Finger-like structure persists at higher frequencies.

An X-shape structure emerges at increased frequency.

Structures are signatures of non-sequential double ionization.

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.