The interpretation of X-ray absorption spectra of water and ice

The changes in the X-ray Absorption Spectrum (XAS) of water upon formation of hydrogen bonds (H-bonds) are analyzed with the aid of Density Functional Theory (DFT) calculations. The tetrahedral symmetry of the ice structure removes the p-character from the 4a(1) level, leading to very weak intensity in the pre-edge region. Breaking an accepting H-bond has very little effect on the absorption spectrum. A broken donating H-bond, however, is identified through a strong pre-edge feature in XAS. The asymmetry on the hydrogen side causes a(1) and b(2) orbital mixing and the orbitals localize along the internal O-H bonds. (C) 2002 Elsevier Science B.V. All rights reserved.