量子材料研究セミナーシリーズ

日時：2007年1月18日（木）14:00〜

場所：萌光館

講演者：Prof. Lars. GM. Pettersson
所属：FYSIKUM, Stockholm University

題名：Two-Component Mixture Model of Liquid Water

要旨：
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 tha!
t 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 st!
ructure 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.

[1] Wernet et al, Science 304, 995 (2004)
[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)