Formation of very low energy states crossing the ionization threshold of argon atoms in strong mid-infrared fields

TL;DR

This study investigates very low energy electron states in argon atoms ionized by intense mid-infrared laser pulses, revealing features beyond the strong-field approximation through combined experimental and theoretical analysis.

Contribution

It provides new experimental data and theoretical insights into low-energy electron structures, including the ZES, in strong-field ionization of argon with mid-IR lasers.

Findings

Identification of the zero-energy structure (ZES) in electron spectra.

Explanation of low-energy features via Coulomb focusing and Rydberg state recapture.

Experimental validation of theoretical models for low-energy electron dynamics.

Abstract

Atomic ionization by intense mid-infrared (mid-IR) pulses produces low electron energy features that the strong-field approximation, which is expected to be valid in the tunneling ionization regime characterized by small Keldysh parameters ($\gamma \ll 1$), cannot describe. These features include the low-energy structure (LES), the very-low-energy structure (VLES), and the more recently found zero-energy structure (ZES). They result from the interplay between the laser electric field and the atomic Coulomb field which controls the low-energy spectrum also for small $\gamma$. In the present joint experimental and theoretical study we investigate the vectorial momentum spectrum at very low energies. Using a reaction microscope optimized for the detection of very low energy electrons, we have performed a thorough study of the three-dimensional momentum spectrum well below 1 eV. Our measurements are complemented by quantum and classical simulations, which allow for an interpretation of the LES, VLES and of the newly identified ZES in terms of two-dimensional Coulomb focusing and recapture into Rydberg states, respectively.