Imaging Isotopic Effects in the Enhanced Ionization of Water

A strong enhancement can be achieved in the sequential multiple ionization of water by stretching and unbending the molecule. Selectively ionizing two electrons can induce these geometrical distortions and facilitate the ionization of a third electron. Studying this effect in D₂O, one of the heavier isotopologues of water, we uncovered a critical geometry at which this enhancement is maximal. Here the molecule was entirely unbent and the OD bond lengths were approximately doubled in length (r_OD ~ 2.2 Å). However, the sequential multiple ionization process in water should differ for its other isotopologues, e.g. H₂O and HOD. The width of the nuclear wave packets involved, the speed at which they move, and the paths they take will all differ. By performing a comparative analysis of all three isotopologues and utilizing both experimental data and ab initio theory, we seek to identify how the enhanced ionization effect differs, and identify the key contributing factors. For each isotopologue, we initiate sequential multiple ionization by using 6-fs 800-nm pulse pairs with variable interpulse delay. The first pulse forms the dication; the second pulse forms the trication, and nuclear rearrangement occurs in the time between the two. After formation of the trication, the molecule undergoes a Coulomb explosion and the 3D momentum of each fragment is measured in coincidence and used to reconstruct the molecule’s internuclear geometry.