Fragmentation Pathways of ${\mathrm{H}}^{+}({\mathrm{H}}_2\mathrm{O}{)}_2$ after Extreme Ultraviolet Photoionization

Photofragmentation of the protonated water dimer ${\mathrm{H}}^{+}({\mathrm{H}}_2\mathrm{O}{)}_2$, a fundamental system both in aqueous solutions and gas-phase water clusters, has been studied at 13.8 nm using the Free Electron Laser FLASH in Hamburg. In a crossed-beam experiment using time-resolved, single-molecule fragment imaging, the two-body breakup into ${\mathrm{H}}_2{\mathrm{O}}^{+}+{\mathrm{H}}_3{\mathrm{O}}^{+}$ was found as a prominent fragmentation channel with a kinetic energy release of up to 10 eV. This channel was observed with at least a similar yield as events with stronger fragmentation, producing protons together with neutral fragments and showing an absolute cross section of $(0.5\pm 0.2)\times {10}^{-18}{\mathrm{cm}}^2$.

Figure 1

Schematic sketch of the fragment detection system at TIFF.

Figure 2

Fragment velocities recorded after photodissociation of ${\mathrm{H}}^{+}({\mathrm{H}}_2\mathrm{O}{)}_2$ at 13.8 nm without a local electric potential at the interaction region ($V_c=0$). (a) Longitudinal velocities: upper curve, all recorded fragments; lower curve, only fragments arriving in coincidence with a fragment on the respective other detector (coincidence windows marked by horizontal brackets). Vertical dotted lines mark the beam velocity $v_0=151\mathrm{mm}/\mu \mathrm{s}$. (b) Transversal versus longitudinal velocities. Dotted lines indicate the geometrical detection limits, diamonds mark the beam velocity $v_0$, while solid semicircles show points of constant velocity release $\tilde{v}$ (7.3 and $34\mathrm{mm}/\mu \mathrm{s}$ on DET1 and $4\mathrm{mm}/\mu \mathrm{s}$ on DET2).

Figure 3

Fragment velocities recorded with a local potential of $V_c=+700\mathrm{V}$ applied to the interaction region. (a) Longitudinal velocities for all (upper curve) and for coincident fragments (lower curves) with coincidence windows marked by horizontal brackets. Vertical dotted lines mark the mean velocities expected for the indicated fragment types, as obtained from numerical trajectory simulations. (b) Transversal versus longitudinal velocities. Dotted lines indicate the geometrical detection limits.