Electronic signatures of enhanced ionization in water

Strong-field ionization of molecules leads to atomic motion and redistribution of electron density. In water, double ionization can drive the molecule to stretched geometries that reduce the tunneling barrier for further ionization to the trication, a process known as enhanced ionization. Here, we compare electrons emitted in ionization events leading to both the dication and the trication of water and show that electrons removed at the enhanced ionization geometry have higher momenta than those emitted away from this configuration. Using few-cycle pulse pairs at variable delay, we measure both ionic and electronic fragments on each laser shot with a voltage-switched velocity map imaging apparatus. Covariance mapping correlates electron momenta with the corresponding ionic charge states from which they originated, isolating the electronic contribution of the enhanced ionization step. The measured electron momentum distributions are compared to simple strong-field approximation models to examine the origin of the increased electron momenta observed during the enhanced third ionization step.