Photoelectron energy peaks shift against the radiation pressure in strong field ionization

TL;DR

This paper presents experimental evidence that in strong field ionization, photoelectron energies are affected by radiation pressure, causing directional energy shifts due to nondipole interactions, challenging the traditional view of isotropic photoelectron energies.

Contribution

It provides the first experimental evidence of directional energy shifts in photoelectrons caused by radiation pressure in strong laser fields, highlighting nondipole effects.

Findings

Electrons emitted against light propagation have higher energies.

Electrons emitted along light propagation have lower energies.

Energy shifts are attributed to nondipole photon-electron interactions.

Abstract

The photoelectric effect describes the ejection of an electron upon absorption of one or several photons. The kinetic energy of this electron is determined by the photon energy reduced by the binding energy of the electron and, if strong laser fields are involved, by the ponderomotive potential in addition. It has therefore been widely taken for granted that for atoms and molecules the photoelectron energy does not depend on the electron's emission direction but theoretical studies have questioned this since 1990. Here we provide experimental evidence, that the energies of photoelectrons emitted against the light-propagation direction are shifted towards higher values while those electrons that are emitted along the light-propagation direction are shifted to lower values. We attribute the energy shift to a nondipole contribution from the interaction of the moving electrons with the incident photons.