Magnetic fields alter tunneling in strong-field ionization

TL;DR

This study uses a novel experimental approach with counter-propagating laser pulses to clarify how photon momentum and magnetic fields influence electron tunneling during strong-field ionization, resolving longstanding debates.

Contribution

It provides the first unambiguous experimental evidence of magnetic field effects on tunneling electrons, confirming some theoretical predictions and challenging others.

Findings

Magnetic field influences electron tunneling during ionization

Photon momentum transfer to electrons is experimentally demonstrated

The method improves precision in studying strong-field ionization

Abstract

When a strong laser pulse induces the ionization of an atom, momentum conservation dictates that the absorbed photons transfer their momentum $p_{\gamma}=E_{\gamma}/c$ to the electron and its parent ion. Even after 30 years of studying strong-field ionization, the sharing of the photon momentum between the two particles and its underlying mechanism are still under debate in theory. Corresponding experiments are very challenging due to the extremely small photon momentum ($~10^{-4}$ a.u.) and their precision has been too limited, so far, to ultimately resolve the debate. Here, by utilizing a novel experimental approach of two counter-propagating laser pulses, we present a detailed study on the effects of the photon momentum in strong-field ionization. The high precision and self-referencing of the method allows to unambiguously demonstrate the action of the light's magnetic field on the electron while it is under the tunnel barrier, confirming theoretical predictions, disproving others. Our results deepen the understanding of, for example, molecular imaging and time-resolved photoelectron holography.