Control of concerted back-to-back double ionization dynamics in helium

TL;DR

This paper develops and compares four control methods to optimize laser fields for inducing back-to-back double ionization in helium, revealing a classical two-step mechanism underlying this process.

Contribution

It introduces four distinct control procedures to achieve and analyze back-to-back double ionization in helium, a previously challenging target.

Findings

All control methods successfully produce back-to-back electron ejection.

The mechanism involves initial displacement by the field followed by classical Coulomb interactions.

The process is confirmed to be essentially classical through quasi-classical calculations.

Abstract

Double ionization (DI) is a fundamental process that despite its apparent simplicity provides rich opportunities for probing and controlling the electronic motion. Even for the simplest multielectron atom, helium, new DI mechanisms are still being found. To first order in the field strength, a strong external field doubly ionizes the electrons in helium such that they are ejected into the same direction (front-to-back motion). The ejection into opposite directions (back-to-back motion) cannot be described to first order, making it a challenging target for control. Here, we address this challenge and optimize the field with the objective of back-to-back double ionization using a (1 + 1)-dimensional model. The optimization is performed using four different control procedures: (1) short-time control, (2) derivative-free optimization of basis expansions of the field, (3) the Krotov method, and (4) control of the classical equations of motion. All four procedures lead to fields with dominant back-to-back motion. All the fields obtained exploit essentially the same two-step mechanism leading to back-to-back motion: first, the electrons are displaced by the field into the same direction. Second, after the field turns off, the nuclear attraction and the electron-electron repulsion combine to generate the final motion into opposite directions for each electron. By performing quasi-classical calculations, we confirm that this mechanism is essentially classical.