Strong Field Non-Franck-Condon Ionization of H$_2$: A Semi-Classical Analysis

TL;DR

This study uses a semi-classical approach to analyze strong-field ionization of H$_2$ molecules, revealing enhanced ionization with circular polarization and non Franck-Condon vibrational distributions in the resulting cation.

Contribution

It introduces a semi-classical method to investigate how laser polarization and wavelength influence ionization and vibrational states in H$_2$, highlighting non Franck-Condon effects.

Findings

Increased ionization efficiency with circular polarization.

Vibrational distributions favor lower-lying levels than Franck-Condon predictions.

Non Franck-Condon vibrational distributions observed in both linear and circular polarizations.

Abstract

Single ionization of H$_2$ molecules exposed to strong and short laser pulses is investigated by a semi-classical method. Three laser characteristics are considered: i) The carrier-wave frequency corresponds to wavelengths covering and bridging the two ionization regimes: From tunnel ionization (TI) at 800 nm to multiphoton ionization (MPI) at 266 nm. ii) Values of the peak intensity are chosen within a window to eliminate competing double ionization processes. iii) Particular attention is paid to the polarization of the laser field, which can be linearly or circularly polarized. The results and their interpretation concern two observables, namely the end-of-pulse total ionization probability and vibrational distribution generated in the cation H$_2^+$. The most prominent findings are an increased ionization efficiency in circular polarization and a vibrational distribution of the cation that favors lower-lying levels than those that would be populated in a vertical (Franck-Condon) ionization, leading to non Franck-Condon distributions, both in linear and circular polarizations.