Analytical approach to Coulomb focusing in strong field ionization

TL;DR

This paper develops an analytical classical theory to understand Coulomb focusing effects in strong field ionization, revealing how rescattering influences photoelectron momentum distributions beyond the dipole approximation.

Contribution

It introduces a perturbative analytical approach to calculate Coulomb momentum transfer, including high-order rescattering effects, in the non-dipole regime of strong laser fields.

Findings

Analytical formulas accurately describe Coulomb momentum transfer.

High-order rescatterings significantly affect the Coulomb focusing cusp.

The energy-dependent bend of the cusp is explained and its scaling analyzed.

Abstract

The role of the Coulomb potential of the atomic core for creation of caustics in the photoelectron momentum distribution for tunneling ionization in a linearly polarized strong laser field, usually termed as Coulomb focusing, is investigated within classical theory beyond the dipole approximation. Coulomb focusing is addressed by analytical calculation of Coulomb momentum transfer to the tunneled electron due to rescatterings, while applying perturbation theory and classifying the recollisions either as fast or as slow. The accuracy of the obtained analytical formulas for the Coulomb momentum transfer is investigated by analytical derivation of asymptotic photoelectron momentum distribution and its comparison to exact numerical calculations. With the help of the applied analytical treatment, we analyze the origin of the counterintuitive energy-dependent bend of the Coulomb focusing cusp in the photoelectron momentum distribution in a linearly polarized laser field in the non-dipole regime, and its scaling with the field parameters. The importance of high-order recollisions is also investigated and they are shown to be responsible for a decrease of the bend of the cusp at very low energies in this regime. In general, the momentum transfer caused by high-order rescattering events, while being a perturbation with respect to the instantaneous electron momentum at the rescattering, is shown to induce a significant non-perturbative contribution to the total momentum transfer.