Universal features in sequential and nonsequential two-photon double ionization of helium

TL;DR

This paper reveals universal features in two-photon double ionization of helium, showing that energy spacing between emitted electrons governs distributions across both nonsequential and sequential regimes, highlighting a continuous transition.

Contribution

It demonstrates the universal energy and angular distribution features in helium double ionization, independent of photon energy, using fully correlated ab initio calculations.

Findings

Energy distribution follows a universal shape function.

Angular distributions are highly sensitive to photon energy.

Shake-up interferences significantly affect angular patterns.

Abstract

We analyze two-photon double ionization of helium in both the nonsequential and sequential regime. We show that the energy spacing between the two emitted electrons provides the key parameter that controls both the energy and the angular distribution and reveals the universal features present in both the nonsequential and sequential regime. This universality, i.e., independence of photon energy, is a manifestation of the continuity across the threshold for sequential double ionization. For all photon energies, the energy distribution can be described by a universal shape function that contains only the spectral and temporal information entering second-order time-dependent perturbation theory. Angular correlations and distributions are found to be more sensitive to the photon energy. In particular, shake-up interferences have a large effect on the angular distribution. Energy spectra, angular distributions parameterized by the anisotropy parameters, and total cross sections presented in this paper are obtained by fully correlated time-dependent ab initio calculations.