Speaker
Prof.
Yoshihisa Harada
(University of Tokyo)
Description
X-ray and neutron diffraction techniques have long been used
as a direct probe of the hydrogen bond network of liquid
water, while in this decade X-ray spectroscopic techniques
have been widely used with the development of high
brilliance synchrotron radiation source. X-ray absorption
(XAS) and emission (XES) spectra of liquid water are one of
the recently debated approaches to determine the local
hydrogen bond network of liquid water through observation of
its local valence electronic structure [1]. Here we review
our X-ray emission technique and spectroscopic results
including temperature [2], isotope [3] and polarization [4]
dependence by referring those debates. In addition, recent
high resolution O 1s resonant inelastic X-ray scattering
(RIXS) spectra add information about vibrational energy in
the OH-stretching mode. Figure 1 shows a comparison of the
energy separation in the RIXS spectrum at XAS pre-edge
excitation of liquid H2O water with the OH stretching mode
by Raman spectroscopy [5]. The RIXS spectrum is blue shifted
because water molecules with a highly weakened or broken
donating hydrogen bond are selected by the pre-edge
excitation. We will also show evidence of this selectivity
by excitation energy dependence of the RIXS vibrational
structure, which strongly supports the interpretation of our
O 1s XAS/XES results in terms of a mixture
(micro-heterogeneity) model, where the network is considered
as a mixture of various hydrogen bond configurations. The
‘vibrational RIXS’ technique bridges hydrogen bond
configuration and the electronic structure of water, which
can be applied to a wide range of solutions in the near future.
[1] A. Nilsson and L. G. M. Pettersson, Chem. Phys. 389, 1
(2011).
[2] T. Tokushima, Y. Harada, O. Takahashi, Y. Senba, H.
Ohashi, L. G. M. Pettersson, A. Nilsson, and S. Shin, Chem.
Phys. Lett. 460, 387 (2008).
[3] T. Tokushima, Y. Harada, Y. Horikawa, O. Takahashi, Y.
Senba, H. Ohashi, L. G. M. Pettersson, A. Nilsson, and S.
Shin, J. Electron Spectrosc. Relat. Phenom. 177, 192 (2010).
[4] T. Tokushima, Y. Horikawa, H. Arai, Y. Harada, O.
Takahashi, L. G. M. Pettersson, A. Nilsson, S. Shin, J.
Chem. Phys. 136, 044517 (2012).
[5] Y. Harada, T. Tokushima, Y. Horikawa, O. Takahashi, H.
Niwa, M. Kobayashi, M. Oshima, Y. Senba, H. Ohashi, K.
Wikfeldt, A. Nilsson, L. G. M. Pettersson, S. Shin, Phys.
Rev. Lett. 111, 193001 (2013).
