Imaging Molecular Motion During the Strong-Field Enhanced Ionization of Water

There exists an enhancement in the strong-field (multiple) ionization (SFI) of water that leads to an increased trication (H₂O³⁺) yield at certain “critical” internuclear geometries. Here we investigate this enhancement by studying the three-body dissociations of the trication and dication. To produce these charge states, we performed SFI using two schemes: 1) 800-nm pulses of variable duration, and 2) 6-fs 800-nm pulse pairs of variable interpulse delay. In scheme 1, we found that the ratio of triply to doubly charged three-body dissociations increases exponentially with pulse duration from 5 to 20 fs (at constant peak intensity). This trend suggests that longer durations allow the molecule more time to distort within the field and reach the critical geometry. In scheme 2, we identify a similar, though smaller, enhancement in this ratio at particular delays. This observation indicates that the critical geometry may be reached via field-free internuclear motion manifesting in the time between formation of the dictation and the arrival of the second pulse. In either scheme, after formation of the trication, the molecule undergoes a Coulomb explosion. The 3D momentum of each resulting fragment is measured in coincidence and used to reconstruct the molecule’s internuclear geometry.