Near-threshold high-order harmonic spectroscopy with aligned molecules

TL;DR

This paper investigates high-order harmonic generation in aligned molecules near the ionization threshold, revealing two distinct signal contributions influenced by molecular alignment and ellipticity, and extends spectroscopy into the near-threshold range.

Contribution

It identifies two different mechanisms behind harmonic signals near the ionization threshold and incorporates Coulomb effects into classical electron trajectory analysis.

Findings

Two harmonic contributions respond differently to molecular alignment.

Coulomb potential significantly influences electron dynamics.

High harmonic spectroscopy can be applied near the ionization threshold.

Abstract

We study high-order harmonic generation in aligned molecules close to the ionization threshold. Two distinct contributions to the harmonic signal are observed, which show very different responses to molecular alignment and ellipticity of the driving field. We perform a classical electron trajectory analysis, taking into account the significant influence of the Coulomb potential on the strong-field-driven electron dynamics. The two contributions are related to primary ionization and excitation processes, offering a deeper understanding of the origin of high harmonics near the ionization threshold. This work shows that high harmonic spectroscopy can be extended to the near-threshold spectral range, which is in general spectroscopically rich.