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Liquid Water

Many aspects of the microscopic structure of water in its many phases are still only incompletely understood. Combining DFT with core-level spectroscopies we are investigating the local electronic structure of bulk liquid water, which is substantially different from the one of the solid phase and the vapor.
For ex. we have recently demonstrated that X-ray absorption (XAS) and X-ray Raman scattering (XRS) measurements of ice and liquid water show major differences in the spectral profiles. There is a well defined pre-edge structure that can be seen in the spectrum of the liquid. The liquid water XA spectrum closely recalls the one of the ice surface where a dominant fraction of molecules in the first half-bilayer has one free O-H group.
We interpret this finding with the aid of DFT calculations and associate the intensity in the pre-edge region to water molecules with broken donating H-bonds. The asymmetry on the hydrogen side of the water molecule causes a1 and b2 orbital mixing and the orbitals localize along the internal O-H bonds. Our work indicates that most molecules in liquid water are in two-hydrogen-bonded configurations, possibly arranged in strongly H-bonded chains or rings, in contrast with the four-hydrogen-bonded tetrahedral structure in ice. Upon heating from 25 to 90 C circa 10% of the molecules changes from tetrahedral environment to two-H-bonding configurations. These conclusions are consistent with neutron and X-ray diffraction data and recent femto-second IR spectroscopy findings although serious discrepancies with structures based on current molecular dynamics (MD) simulations are observed.
The nature of hydrogen bond in water is also studied by mean of XES and XPS, always coupled with theoretical calculations, and new techniques in the theoretical analysis of the spectra are developed in the group. For. ex. we are implementing a technique to simulate core-level spectroscopies within the Car-Parrinello ab-initio MD framework that will allow the study of the excitation induced dynamics.
REFERENCES:
M. Cavalleri, M. Odelius, A. Nilsson and L.G.M. Pettersson, "X-ray absorption spectra of water within a plane-wave Car-Parrinello molecular dynamics framework" J. Chem. Phys., 121(20), 10065-10075 (2004).
Ph. Wernet, D. Nordlund, U. Bergman, M. Cavalleri, M. Odelius, H. Ogasawara, L.Å. Näslund, T.K. Hirsh, L. Ojamäe, P. Glatzel, L.G.M. Pettersson and A. Nilsson, "The structure of the first coordination shell in liquid water" Science, 304(5673), 995-999 (2004), published online 01 April 04, 10.1126/science.1096205.
S. Myneni, Y. Luo, L.Å Näslund, M. Cavalleri, L. Ojamäe, H. Ogasawara, A. Pelmenschikov, P. Väterlein, C. Heske, L.G.M. Pettersson, and A. Nilsson,J. of Physics-CM., 14, 8, L213-L219 (2002)
M.Cavalleri, H.Ogasawara, L.G.M. Pettersson and A. Nilsson,Chem.Phys.Lett., 364/3-4 pp. 363-370 (2002)
U. Bergmann, Ph. Wernet, P. Glatzel, M. Cavalleri, L.G.M. Pettersson, M. Odelius, P.-C. Lin, A. Nilsson and S.P. Cramer,PRB, 66, 092107 (2002)
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