Electron correlation in two-photon double ionization of helium from attosecond to FEL pulses

TL;DR

This study explores how electron correlation affects two-photon double ionization of helium using ultrashort XUV pulses, revealing interference effects that depend on pulse duration and could help measure FEL pulse durations.

Contribution

It provides the first detailed analysis of electron correlation effects in two-photon double ionization across a range of ultrashort pulse durations, highlighting interference phenomena.

Findings

Electron correlation influences angular distributions of ejected electrons.

Double ionization probabilities vary non-uniformly with pulse duration.

Interference between direct and indirect ionization pathways depends on pulse duration.

Abstract

We investigate the role of electron correlation in the two-photon double ionization of helium for ultrashort XUV pulses with durations ranging from a hundred attoseconds to a few femtoseconds. We perform time-dependent ab initio calculations for pulses with mean frequencies in the so-called "sequential" regime (photon energy above 54.4 eV). Electron correlation induced by the time correlation between emission events manifests itself in the angular distribution of the ejected electrons, which strongly depends on the energy sharing between them. We show that for ultrashort pulses two-photon double ionization probabilities scale non-uniformly with pulse duration depending on the energy sharing between the electrons. Most interestingly we find evidence for an interference between direct ("nonsequential") and indirect ("sequential") double photo-ionization with intermediate shake-up states, the strength of which is controlled by the pulse duration. This observation may provide a route toward measuring the pulse duration of FEL pulses.