Decoding sequential vs non-sequential two-photon double ionization of helium using nuclear recoil

TL;DR

This paper demonstrates that nuclear recoil momentum imaging can reveal signatures of sequential two-photon double ionization in helium, even below the energy threshold, offering a new experimental approach without needing coincident electron detection.

Contribution

It introduces a method to detect sequential ionization signatures via nuclear recoil imaging, expanding experimental capabilities for studying multi-photon ionization processes.

Findings

Nuclear recoil cross section shows signatures of sequential ionization.

Reveals process signatures below the sequential threshold.

Proposes a viable experimental technique with future light sources.

Abstract

Above 54.4 eV, two-photon double ionization of helium is dominated by a sequential absorption process, producing characteristic behavior in the single and triple differential cross sections. We show that the signature of this process is visible in the nuclear recoil cross section, integrated over all energy sharings of the ejected electrons, even below the threshold for the sequential process. Since nuclear recoil momentum imaging does not require coincident photoelectron measurement, the predicted images present a viable target for future experiments with new short-pulse VUV and soft X-ray sources.