A numerical study of two-photon ionization of helium using the Pyprop framework

TL;DR

This paper introduces a new numerical framework to study two-photon double ionization of helium, providing detailed calculations of cross sections and analyzing the convergence near the ionization threshold.

Contribution

The paper presents a novel ab initio six-dimensional time-dependent Schrödinger equation solver for helium ionization, addressing debates on nonsequential double ionization mechanisms.

Findings

Cross sections increase near the threshold energy.

The rise in cross section is not solely due to sequential ionization.

The convergence of the cross section depends on laser pulse duration.

Abstract

Few-photon induced breakup of helium is studied using a newly developed ab initio numerical framework for solving the six-dimensional time-dependent Schroedinger equation. We present details of the method and calculate (generalized) cross sections for the process of two-photon nonsequential (direct) double ionization at photon energies ranging from 39.4 to 54.4 eV, a process that has been very much debated in recent years and is not yet fully understood. In particular, we have studied the convergence property of the total cross section in the vicinity of the upper threshold (54.4 eV), versus the pulse duration of the applied laser field. We find that the cross section exhibits an increasing trend near the threshold, as has also been observed by others, and show that this rise cannot solely be attributed to an unintended inclusion of the sequential two-photon double ionization process, caused by the bandwidth of the applied field.