Bond rearrangement during Coulomb explosion of water molecules

Bond rearrangement, namely the dissociation of water ions into ${{\mathrm{H}}_2}^{+}+{\mathrm{O}}^{(q-1)+}$ ($q=1$–4) following fast ion-impact ionization, unexpectedly occurs following multiple ionization of water in spite of the presumably fast “Coulomb explosion” of the transient molecular ion. Furthermore, the branching ratio of bond rearrangement is found to be nearly equal for each level of ionization, $q$. In addition, formation of ${{\mathrm{H}}_2}^{+}$ is more than twice as likely to occur from the lighter water isotopologue ${\mathrm{H}}_2{\mathrm{O}}^{+}$ than ${{\mathrm{D}}_2}^{+}$ from ${\mathrm{D}}_2{\mathrm{O}}^{+}$. These findings are consistent with the ground state dissociation mechanism in which a fast projection of the ground state nuclear wave function onto the vibrational continuum of the cation potential energy surface is sometimes followed by ${{\mathrm{H}}_2}^{+}$ formation.

Figure 1

The yield of ${{\mathrm{H}}_2}^{+}$ relative to ${\mathrm{H}}_2{\mathrm{O}}^{+}$ (and isotopologues) as a function of the projectile velocity. Proton impact—open symbols, 1 MeV/amu ${\mathrm{F}}^{9+}$ impact—half-full (magenta) symbol, and electron-impact data (from Ref. [10])—full symbols (see text). The error bars are at a $1\sigma$ level.

Figure 2

A contour plot of a cut of the water cation ground state potential energy surface (keeping $\beta ={90}^{\circ }$), which is identical for the three water isotopologues within the Born-Oppenheimer approximation. The internuclear distances $R_{Oz}$—oxygen-proton or oxygen-deuteron distance, and $R_{zz}$—the distance between the two light nuclei (see text) are shown in the inset of panel (a). The square of the nuclear wave function of the ground state of neutral water is plotted on the PES as a linear-scale color-filled contour plot. The spatial extent of the ground state nuclear wave function increases with lighter hydrogen nuclei. Panel (a) shows the situation for ${\mathrm{D}}_2\mathrm{O}$ and (b) for ${\mathrm{H}}_2\mathrm{O}$.

Figure 3

The (a) raw, (b) random, and (c) true coincidence time-of-flight (TOF) spectra of ${\mathrm{H}}_2\mathrm{O}$ ionized by 1 MeV/amu ${\mathrm{F}}^{7+}$ collisions (ch., channel – arbitrary time unit for the TOF window of interest). The labeled peaks are: (d) ${\mathrm{H}}^{+}+{\mathrm{OH}}^{+}$; (e) ${\mathrm{H}}^{+}+{\mathrm{O}}^{+}$; (f) ${\mathrm{H}}^{+}+{\mathrm{O}}^{2+}$; (g) ${{\mathrm{H}}_2}^{+}+{\mathrm{O}}^{+}$; (h) ${{\mathrm{H}}_2}^{+}+{\mathrm{O}}^{2+}$; (i) ${{\mathrm{H}}_2}^{+}+{\mathrm{O}}^{3+}$.

Figure 4

Coincidence time-of-flight (CTOF) spectra of water molecules ionized by 1 MeV/amu ${\mathrm{F}}^{7+}$ collisions. The main two-body breakup, ${\mathrm{H}}^{+}+{\mathrm{OH}}^{+}$, is shown for comparison. Note that the spread of events perpendicular to the lines is caused by our timing resolution, which was limited by instabilities in bunching over the 11 hours of measurement.