Strong field ionisation of Argon: Electron momentum spectra and nondipole effects

TL;DR

This study explores how relativistic nondipole effects influence electron momentum spectra during strong field ionisation of argon, revealing energy-dependent asymmetries and underlying dynamics through experiments and simulations.

Contribution

It provides the first detailed experimental and theoretical analysis of nondipole effects on argon ionisation at low electron energies using a reaction microscope and TDDE simulations.

Findings

Nondipole effects cause measurable asymmetries in electron spectra.

Simulation accurately reproduces experimental asymmetries.

Classical analysis explains the origin of negative momentum shifts.

Abstract

We investigate the influence of relativistic nondipole effects on the photoelectron spectra of argon, particularly in the low kinetic energy region (0 eV - 5 eV). In our experiment, we use intense linearly polarised 800 nm laser pulse to ionise Ar from a jet and we record photoelectron energy and momentum distributions using a reaction microscope (REMI). Our measurements show that nondipole effect can cause an energy-dependent asymmetry along the laser propagation direction in the photoelectron energy and momentum spectra. Model simulation based on time-dependent Dirac equation (TDDE) can reproduce our measurement results. The electron trajectory analysis based on classical model reveals that the photoelectron which obtains negative momentum shift along laser propagation direction is caused by the interplay between the Lorenz force induced radiation pressure during its free propagation in continuum and re-scattering by Coulomb potential of the parent ion when it is driven back by the laser field.