Ionization in Orthogonal Two-Color Laser Fields - Origin and Phase Dependence of Trajectory-Resolved Coulomb Effects

TL;DR

This study investigates how Coulomb effects influence electron momentum distributions during ionization in orthogonal two-color laser fields, revealing that Coulomb focusing is surprisingly unaffected by weak second harmonic fields due to classical trajectory dynamics.

Contribution

The paper demonstrates, through experiments and simulations, that Coulomb focusing in strong OTC laser fields is largely insensitive to weak streaking fields, highlighting a nuanced interplay of initial electron momentum and Coulomb effects.

Findings

Coulomb focusing appears as a cusp in electron momentum spectra.

The Coulomb focused structure is nearly unaffected by weak second harmonic fields.

Classical trajectory simulations explain the phase insensitivity of Coulomb focusing.

Abstract

We report on electron momentum distributions from single ionization of Ar in strong orthogonally polarized two-color (OTC) laser fields measured with the COLTRIMS technique. We study the effect of Coulomb focusing whose signature is a cusp like feature in the center of the electron momentum spectrum. While the direct electrons show the expected strong dependence on the phase between the two colors, surprisingly the Coulomb focused structure is almost not influenced by the weak second harmonic streaking field. This effect is explained by the use of a CTMC simulation which describes the tunneled electron wave packet in terms of classical trajectories under the influence of the combined Coulomb- and OTC laser field. We find a subtle interplay between the initial momentum of the electron upon tunneling, the ionization phase and the action of the Coulomb field that makes the Coulomb focused part of the momentum spectrum apparently insensitive to the weaker streaking field.