Topics List

Quantum Materials Research Seminar Series

Date and time: Jan 18 (Thu) 14:00-
Place: Houkoukan

Speaker:Prof. Lars. GM. Pettersson
Affiliation:FYSIKUM, Stockholm University
Title:Two-Component Mixture Model of Liquid Water

The measured electronic structure of liquid water shows clear evidence of specific species in the liquid [1,2]. 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 [1]. Through spectrum simulations and model experiments (bulk and surface of ice) we show that the specific features in the liquid XA 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., 3-10]. New X-ray Emission (XE) data show two distinct 1b1 features which interconvert with temperature [2]. Excitation energy and temperature dependence and comparison with ice demonstrate that the high-energy 1b1 peak is associated with single-donor (SD) species while the low-energy peak with tetrahedrally bonded molecules. The lack of broadening and new features at higher temperature rule out continuum models for water and instead provide strong support for a mixture model of predominantly strongly asymmetric species with a minority tetrahedrally coordinated. Neutron and x-ray diffraction have been of particular importance in determining the structure of the liquid and have provided radial distribution functions (RDF) used in calibrating simulation models. The present accepted "experimental" O-O RDF is, however, not from experiment but taken from a TIP4P-pol2 simulation which was in best agreement with the experimental I(Q). New, more extended x-ray diffraction data show significant new structure in the Fourier transform giving the first, second and third coordination shells [11]. We use Reverse Monte Carlo (RMC) [12] modeling to show that this structure is consistent with neutron diffraction data and fit a range of water models in a search for water models that are able to reproduce established properties of water as well as new XAS, XES and diffraction data.

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[2] Tokushima et al., to be published.
[3] Nilsson et al, Science 308, 793 (2005)
[4] Hetenyi et al, J. Chem. Phys., 120, 8632 (2004)
[5] Cavalleri et al, Phys. Chem. Chem. Phys. 7, 2854 (2005)
[6] Naslund et al, J. Phys. Chem. B 109, 13835 (2005)
[7] Smith et al, Proc. Natl. Acad. Sci. USA 102, 14171 (2005)
[8] Odelius et al, Phys. Rev. Letters 94, 227401 (2005)
[9] Head-Gordon and Johnson, Proc. Natl Acad. Sci. (USA) 103, 7973 (2006)
[10] Smith et al, Science, 306, 851 (2004)
[11] Fu et al., to be published.
[12] McGreevy, J. Phys.: Condens. Matter 13, R877 (2001)