Laser-driven recollisions under the Coulomb barrier

TL;DR

This paper investigates the discrepancies between ab initio quantum simulations and the strong-field approximation in photoelectron spectra, revealing how Coulomb-corrected recollisions under the barrier significantly influence ionization probabilities.

Contribution

It introduces a Coulomb-corrected quantum orbit framework that accounts for soft recollisions under the Coulomb barrier, improving understanding of ionization dynamics.

Findings

Enhanced ionization probabilities due to Coulomb recollisions.

Conditions under which SFA predictions are accurate.

Role of complex-time recollisions in quantum paths.

Abstract

Photoelectron spectra obtained from the ab initio solution of the time-dependent Schr\"odinger equation can be in striking disagreement with predictions by the strong-field approximation (SFA) not only at low energy but also around twice the ponderomotive energy where the transition from the direct to the rescattered electrons is expected. In fact, the relative enhancement of the ionization probability compared to the SFA in this regime can be several orders of magnitude. We show for which laser and target parameters such an enhancement occurs and for which the SFA prediction is qualitatively good. The enhancement is analyzed in terms of the Coulomb-corrected action along analytic quantum orbits in the complex-time plane, taking soft recollisions under the Coulomb barrier into account. These recollisions in complex time and space prevent a separation into sub-barrier motion up to the "tunnel exit" and subsequent classical dynamics. Instead, the entire quantum path up to the detector determines the ionization probability.