Probing the Electron Delocalization in Liquid Water and Ice at Attosecond Time Scales

We determine electron delocalization rates in liquid water and ice using core-hole decay spectroscopy. The hydrogen-bonded network delocalizes the electrons in less than 500 as. Broken or weak hydrogen bonds—in the liquid or at the surface of ice—provide states where the electron remains localized longer than 20 fs. These asymmetrically bonded water species provide electron traps, acting as a strong precursor channel to the hydrated electron.

Figure 1

Auger-electron decay processes used to derive site-specific time scales in this work. (Top) Normal Auger decay (left), where the excited electron is delocalized from the decaying atom and spectator Auger decay (right), where the excited electron remains localized during the decay. (Bottom) Simulated spectra of a cluster model (see text) indicate a shift of $\sim 5\mathrm{eV}$ between the two decay channels due to screening by the spectator electron.

Figure 2

O $1s$ x-ray absorption spectra (insets) of liquid water (A) and ice (B) with corresponding O $KLL$ Auger decay spectra for selective excitations. Using the decay from nonresonant excitation at 550(548) eV as model spectra for normal Auger decay, the normal Auger fraction ($f_{\mathrm{Aug}}$) is quantified for the excitation at the postedge ($\sim 541\mathrm{eV}$) and at the preedge ($\sim 535\mathrm{eV}$). The difference between the measured spectra (line) and the normal Auger fraction (dotted) is associated with the spectator decay spectra (dashed), shifted to $\sim 5\mathrm{eV}$ higher kinetic energy in agreement with theory (see Fig. 1). All spectra were area normalized in the region 495–510 eV prior to the quantification.

Figure 3

Orbital plots of core-excited states in (a) the postedge region and (b) the preedge region compared to (c) the solvated electron, represented by the relaxed HOMO orbital of a $-1e$ charged cluster of six water molecules oriented around a central cavity.

Figure 4

Schematic diagram of electron trapping in liquid water: (1 & 2) an excess electron propagating along the H-bond network, (3) trapping at a weakly H-bonded OH group of a single-donor site followed by (4) electron solvation dynamics to reach the solvated electron (5).