Sub-barrier pathways to Freeman resonances

TL;DR

This paper presents a theoretical model explaining Freeman resonances in strong-field ionization, highlighting the role of sub-barrier recollision and interference effects, with implications for enhancing ionization processes.

Contribution

It introduces a perturbative SFA-based model that elucidates the mechanism of intermediate excited state population during Freeman resonances, offering new insights into electron dynamics.

Findings

Constructive interference enhances ionization at Freeman resonances.

Sub-barrier recollision facilitates intermediate excited state population.

Model provides intuitive understanding and potential control methods.

Abstract

The problem of Freeman resonances [R. R. Freeman \textit{et al.}, Phys. Rev. Lett. \textbf{59}, 1092 (1987)] when strong field ionization is enhanced due to the transient population of excited states during the ionization, is revisited. An intuitive model is put forward which explains the mechanism of the intermediate population of excited states during nonadiabatic tunneling ionization via the under-the-barrier recollision and recombination. The theoretical model is based on perturbative strong-field approximation (SFA), where the sub-barrier bound-continuum-bound pathway is described in the second-order SFA, while the further ionization from the excited state by an additional perturbative step. The enhancement of ionization is shown to arise due to the constructive interference of contributions into the excitation amplitudes originating from different laser cycles. The applied model provides an intuitive understanding of the electron dynamics during a Freeman resonance in strong-field ionization, as well as means of enhancing the process and possible applications to related processes.