KTH/Nordita/SU seminar in Theoretical Physics

The Local Structure of Water

by Prof. Lars Pettersson (Chemical Physics)

We have investigated the electronicstructure of water and ice using a combination of experimental and theoretical techniques [1]. The X-ray Absorption (XA) spectrum of the liquid is distinctly different from that of tetrahedrally coordinated bulk ice, where the liquid shows a distinct pre-edge feature and a strong enhancement of the intensity at the edge. Through spectrum simulations and model experiments (bulk and surface of ice) we show that the specific features in the liquid spectrum are due exclusively to asymmetric configurations with only two strong hydrogen bonds: one donating and one accepting; this could be indicative of chain- or ring-like structures in the liquid but is not reproduced by present simulation techniques [1]. This result has caused a heated debate in the literature [e.g., 2-11]. Here I will discuss the interpretation of the spectra on the basis of theoretical modeling and additional experimental data. Neutron and x-ray diffraction have been of particular importance in determining the structure of the liquid. A recent reevaluation of the neutron and x-ray diffraction data by A. Soper [12] shows equally good fit of the data using an asymmetric H-bonding model according to [1] as for the standard tetrahedral model. Using the structures obtained by Soper we, however, show that none of the thus obtained models fit the available XA or IR/Raman data and furthermore have to question standard approaches to connect water structure with IR/Raman spectra. To resolve the situation we are using Reverse Monte Carlo (RMC) [13] modeling to fit a range of water models (hexamer rings, tetrahedral, mixed) to new, more extended x-ray and neutron diffraction data as well as data from Compton scattering experiments.