Strong Field Ionization of Water II: Electronic and Nuclear Dynamics En Route to Double Ionization

TL;DR

This study explores how nuclear motion and electronic couplings influence double ionization in water molecules under strong laser fields, revealing complex dynamics that challenge simple vertical ionization models.

Contribution

It provides experimental evidence and theoretical analysis showing the importance of nuclear and electronic dynamics in strong-field double ionization of water, highlighting mechanisms beyond vertical ionization assumptions.

Findings

Strong intra-pulse dynamics observed in double ionization yields.

Departure from vertical ionization expectations even with 10 fs pulses.

Multiple mechanisms, including nuclear motion and laser-induced couplings, influence ionization.

Abstract

We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D$^{+}$/D$^{+}$/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D$_2$O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data.