Ionization behavior of molecular hydrogen in intense laser fields: Influence of molecular vibration and alignment

TL;DR

This study investigates how molecular vibration and alignment affect the ionization of hydrogen molecules in intense laser fields, revealing the limitations of fixed-nuclei models and proposing a corrected tunneling formula for better predictions.

Contribution

It demonstrates the dependence of ionization on internuclear distance and alignment, clarifies the role of vibrational dynamics, and introduces a frequency-dependent correction to tunneling ionization rates.

Findings

Ionization yield strongly depends on internuclear distance.

Breakdown of fixed-nuclei approximation is not due to vibrational dynamics.

Corrected tunneling formula aligns well with observed ionization rates.

Abstract

The alignment- and internuclear-distance dependent ionization of H$_2$ exposed to intense, ultrashort laser fields is studied by solving the time-dependent two-electron Schr\"odinger equation. In the regime of perturbative few-photon ionization, a strong dependence of the ionization yield on the internuclear distance is found. While this finding confirms a previously reported breakdown of the fixed-nuclei approximation for parallel alignment, a simpler explanation is provided and it is demonstrated that this breakdown is not due to vibrational dynamics during the laser pulse. The persistence of this effect even for randomly aligned molecules is demonstrated. Furthermore, the transition from the multiphoton to the quasi-static (tunneling) regime is investigated considering intense 800 nm laser pulses. While the obtained ionization yields differ significantly from the prediction of Ammosov-Delone-Krainov rates, we find a surprisingly good quantitative agreement after introducing a simple frequency-dependent correction to the standard tunneling formula.