High-resolution soft X-ray photoelectron spectroscopy of liquid water

High-resolution soft X-ray photoelectron spectra of liquid water (H ₂O and D₂O) were measured using a liquid beam photoelectron spectrometer. The 1a₁ (O1s) band and the lowest valence 1b₁ band had single peaks, which is not consistent with the split 1b₁ → 1a₁ of the X-ray emission band of liquid water if the splitting is assumed to originate from level shifts in two different hydrogen bonding structures. The second valence 3a₁ band of liquid water exhibited a flat top implying that two bands exist underneath a broad feature, which is similar to the case of the 3a₁ band of amorphous ice. The energy splitting between the two 3a₁ bands is estimated to be 1.38 eV (H₂O) and 1.39 eV (D₂O). Ab initio calculations suggest that the large splitting of the 3a₁ band is characteristic of water molecules that function as both proton donor and acceptor. The overall result is consistent with the conventional model of a tetrahedral hydrogen-bonding network in liquid water.