Strong-field ionization of water. II. Electronic and nuclear dynamics en route to double ionization

In this work, 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 multicycle laser pulses, strong evidence for intrapulse 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 to 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 wave function en route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate dependence on 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 D2O, 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.