Isotopic effect in bond rearrangement caused by sudden single and multiple ionization of water molecules.

The production of H$_{2}^{+}$ fragments upon dissociation of water molecules involves rearrangements of the molecular bonds. Using fast ions to ionize a water molecule, electrons can be removed on a time scale of 10 attoseconds, thus freezing the nuclear motion. Our earlier experimental results of H$_{2}^{+}$ + O production rate showed a strong isotopic dependence, which is about two times more likely than D$_{2}^{+}$ + O. Recently, we measured a similar ratio for double ionization of water by 1 MeV/amu F$^{7+}$ projectiles. In particular, the H$_{2}$O$^{2+}$ dissociation into H$_{2}^{+}$ + O$^{+}$ is about twice as large as D$_{2}$O$^{2+} \quad \to $ D$_{2}^{+}$ + O$^{+}$. Finally, we have experimental evidence for H$_{2}^{+}$ formation in triple and quadruple ionization. These first observations of bond rearrangement of multiply ionized molecules indicate a sudden mechanism, i.e. a Franck-Condon-like transition, instead of bonding during the slow dissociation.