Ultrafast Molecular Dissociation of Water in Ice

Using x-ray emission and photoemission spectroscopies to measure the occupied valence levels in a thin crystalline ice film, we resolve the ionization-induced dissociation of water in ice on a femtosecond time scale. Isotope substitution confirms proton transfer during the core-hole lifetime in spite of the nonresonant excitation. Through ab initio molecular dynamics on the core-ionized state, the dissociation and spectrum evolution are followed at femtosecond intervals. The theoretical simulations confirm the experimental analysis and allow for a detailed study of the dissociative reaction path.

Figure 1

Experimental valence PE and XE spectra of a crystalline ice film, plotted on a binding energy scale relative to the Pt substrate Fermi level. The three outer-valence orbitals are denoted as for the free molecule. The PE spectrum was taken at 75 eV, and nonresonant excitation ($\sim 540\mathrm{e}\mathrm{V}$) was used for the XE spectra. The latter shows spectra of both ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ ice. An expansion of the XE spectra for the $1b_1$ region is shown in the inset.

Figure 2

Car-Parrinello MD simulations of $\mathrm{O}1s$ core-ionized state proton (deuterium) dynamics for a water molecule in a ${\mathrm{H}}_2\mathrm{O}$ (${\mathrm{D}}_2\mathrm{O}$) ice $I_h$ lattice, averaged for 64 sampled initial conditions. Shown is the H (D) position along the $\mathrm{O}\mathrm{\text{−}}\mathrm{H}\mathrm{O}$ ($\mathrm{O}\mathrm{\text{−}}\mathrm{D}\mathrm{O}$) bond relative to the core-ionized oxygen, measured at 1 fs intervals. At the top of the figure, selected steps in the dynamics are pictured for a core-ionized ${\mathrm{H}}_2\mathrm{O}$ and its nearest neighbors.

Figure 3

Computed x-ray emission spectra as function of time (fs) after core ionization for (a) ${\mathrm{H}}_2\mathrm{O}$ and (b) ${\mathrm{D}}_2\mathrm{O}$ ice $I_h$ plotted on a binding energy scale. The spectra are averaged for 64 trajectories and broadened with 0.7 eV.

Figure 4

Computed PE (top) and XE spectra for ${\mathrm{H}}_2\mathrm{O}$ (solid line) and ${\mathrm{D}}_2\mathrm{O}$ ice $I_h$ (dotted line). The XE spectra are averaged using an exponential decay function (see text).