Probing the tunneling site of electrons in strong field enhanced ionization of molecules

TL;DR

This paper experimentally verifies the physical mechanism behind enhanced molecular ionization in strong laser fields, confirming the up-field atom ionization model through detailed ion momentum measurements.

Contribution

It provides the first experimental proof of the long-standing prediction that the up-field atom in a molecule is the primary site of ionization during strong field interactions.

Findings

Confirmed the up-field atom as the ionization site in molecules.

Clarified the physical picture of enhanced molecular ionization.

Provided experimental evidence resolving a long-standing debate.

Abstract

Molecules show a much increased multiple ionization rate in a strong laser field as compared to atoms of similar ionization energy. A widely accepted model attributes this to the action of the joint fields of the adjacent ionic core and the laser on its neighbor inside the same molecule. The underlying physical picture for the enhanced ionization is that the up-field atom that gets ionized. However, this is still debated and remains unproven. Here we report an experimental verification of this long-standing prediction. This is accomplished by probing the two-site double ionization of ArXe, where the instantaneous field direction at the moment of electron release and the emission direction of the correlated ionizing center are measured by detecting the recoil sum- and relative-momenta of the fragment ions. Our results unambiguously prove the intuitive picture of the enhanced multielectron dissociative ionization of molecules and clarify a long-standing controversy.