Quantum interference in strong-field ionization by a linearly polarized laser pulse explains non-zero tunnel exit momentum*

We theoretically and numerically investigate the liberation of an atomic electron by a linearly polarized single-cycle near-infrared laser pulse with a peak intensity corresponding to tunnel ionization. This process is of fundamental importance in attosecond physics, with open questions regarding e.g. the tunnel exit momentum.

Based on a phase space analysis with the Wigner function [Opt. Commun. 179, 29 (2000)] and by tracking the energy distribution in the instantaneous (Coulomb + laser) potential, we reveal the importance of quantum interference between tunneling and over-the-barrier pathways of escape [Phys. Rev. A 104, L031102 (2021)]. We highlight the Wigner function’s natural connection to the quantum momentum function (QMF), which enables us to use the QMF as the fundamental quantity to represent the true time-dependent quantum dynamics, including quantum interference. We define a suitable value of the QMF as tunnel exit momentum, without any contradiction to energy conservation, and in a good agreement with recent experimental results [Phys. Rev. Lett. 119, 023201 (2017); Phys. Rev. Lett. 122, 183202 (2019)].