Localizing Electron Density during the Enhanced Ionization of Semi-Heavy Water

All of the stable isotopologues of water (H₂O, D₂O, and HOD) are practically indistinguishable in both their atomic arrangement and electronic structure. However, by selectively ionizing two electrons from semi-heavy water (HOD) using strong laser fields, we can induce an asymmetric unbending and stretching motion that alters the electronic structure to favor localization of electron density along one bond more than the other. We have studied this effect as it pertains to the enhanced ionization of water, a phenomenon in which the instantaneous molecular geometry can assist tunneling ionization at particular “critical geometries”. For the symmetric isotopologues, such as H₂O and D₂O, this critical geometry is a fully unbent, linearized, and symmetrically stretched molecule. For HOD, however, the critical geometry is asymmetric. This results in a preferential departure of electrons along the OH bond during enhanced ionization, rather than the OD. By performing a comparative analysis of all three isotopologues and utilizing both experimental data and ab initio theory, we have demonstrated how we can observe asymmetric electron localization and departure for an asymmetrically heavy isotopologue. 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.