Strong field double ionization: What is under the "knee"?

TL;DR

This paper investigates the phase-space dynamics behind the characteristic 'knee' shape in double ionization probability of helium in strong laser pulses, clarifying the roles of correlated and uncorrelated processes and predicting key intensity thresholds.

Contribution

It introduces a phase-space analysis that reveals the dynamical structures governing double ionization, enhancing understanding beyond the recollision model.

Findings

Identification of dynamical structures regulating ionization

Predictions of intensity thresholds for qualitative changes

Clarification of recollision and core electron roles

Abstract

Both uncorrelated ("sequential") and correlated ("nonsequential") processes contribute to the double ionization of the helium atom in strong laser pulses. The double ionization probability has a characteristic "knee" shape as a function of the intensity of the pulse. We investigate the phase-space dynamics of this system, specifically by finding the dynamical structures that regulate the ionization processes. The emerging picture complements the recollision scenario by clarifying the distinct roles played by the recolliding and core electrons. Our analysis leads to verifiable predictions of the intensities where qualitiative changes in ionization occur, leading to the hallmark "knee" shape.