SPring-8, the large synchrotron radiation facility

Release Date

11 Aug, 2009

RIKEN
SLAC* National Accelerator Laboratory
Stockholm University

Key research findings
• The inhomogeneity of water is caused by the coexistence of two types of microstructure in water.
• The size of one type of inhomogeneous microstructure, similar to ice, is approximately 1 nm in diameter.
• Clarification of the temperature dependence of the microstructures will lead to an understanding of the role of water in organisms and in chemical reactions.

* SLAC: Stanford Linear Accelerator Center

<Figure>

Fig. 1 Schematic of soft X-ray emission spectroscopy The red circles and dotted circles represent electrons and holes, respectively. When an electron in the inner shell is expelled by soft X-ray irradiation, as shown in the left figure (soft X-ray absorption), a hole (the absence of an electron in an electron orbital) is produced in the inner shell. The hole is unstable and a valence electron involved in the hydrogen bond falls to the empty hole to achieve a stable condition. Soft X-ray emission spectroscopy is a method of analyzing the soft X-rays emitted along with the transition of the electron. The state of the electrons is clarified by observing the intensity distribution of the emitted soft X-ray energy.

Fig. 2 Results of small-angle X-ray scattering (Left) Measurement results for pure water. The dark-green to yellow-green curves indicate results obtained at 7, 11, 16, 20, 25, 29, 38, 47, 56, and 74°C. (Right) Measurement results for liquid carbon tetrachloride (CCl4). The dark-green to red curves indicate results obtained at 6, 11, 16, 21, 25, and 30°C. S(Q) on the y-axis is called the structure factor, a quantity corresponding to scattering intensity. Q on the x-axis is called the scattering vector, a quantity corresponding to the scattering angle, i.e., the size of the microstructure. The larger the scattering angle, the smaller the size of the microstructure, and vice versa.

Fig. 3 Soft X-ray emission spectra and X-ray Raman scattering spectra of deuterated water (D2O) According to the results of conventional studies, peak A corresponds to icelike microstructures and peak B corresponds to a group of water molecules with distorted hydrogen bonds. In the inset above the soft X-ray emission spectra, the change in the energy difference between peaks A and B with increasing temperature is shown. In the inset above the X-ray Raman scattering spectra, the change in the energy of peak B with temperature as a parameter is shown in an enlargement of the original spectra. From the spectra, the structure of the icelike microstructures remains unchanged with increasing temperature; however, that of the group of water molecules with distorted hydrogen bonds changes with increasing temperature.

Fig. 4 Simulation using experimentally obtained soft X-ray emission spectra, X-ray Raman scattering spectra, and small-angle X-ray scattering spectra (upper) The two peaks obtained by the previous analysis of emission spectra of pure water (H2O) at room temperature are compared with the two peaks, discovered in this study, for the icelike microstructures and the group of water molecules with distorted hydrogen bonds. (lower) The different microstructures in water are shown by color. The structure is actually three-dimensional, but it is drawn two dimensionally for simplicity. When the water temperature increases, the density in the high-density component decreases and approaches that in the low-density component, leading to the indistinct dot pattern. When this condition is observed by small-angle X-ray scattering, it is difficult to distinguish the two peaks corresponding to the high- and low-density components. The structure of the water under this condition changes continuously through the repeated recombination of hydrogen bonds, which occurs at an interval of 1-2 ps. It is expected that changes in structure can be detected using observation methods that can detect phenomena occurring within less than one femtosecond, which is quicker than the recombination speed of the hydrogen bond, such as soft X-ray emission spectroscopy, X-ray Raman scattering spectroscopy, and small-angle X-ray scattering spectroscopy used in this study.

<Glossary>

*1 SPring-8, a large synchrotron radiation facility, and Stanford Synchrotron Radiation Lightsource (SSRL)
SPring-8 is a RIKEN facility located at Harima Science Garden City, Hyogo Prefecture, that can provide the most powerful synchrotron radiation available to date. The name SPring-8 is derived from Super-Photon ring-8 GeV. Synchrotron radiation is a narrow, powerful beam of electromagnetic radiation generated when electron beams, accelerated to nearly the speed of light, are forced to travel in a curved path by an electromagnet. Studies conducted at SPring-8 using synchrotron radiation include those on nanotechnology, biotechnology, and industrial applications. SSRL is a synchrotron radiation facility established in the SLAC National Accelerator Laboratory (SLAC) in the US.

*2 Joint research group
The joint research team mainly consisted of Shik Shin, team leader, Takashi Tokushima, a research scientist, Yuka Horikawa, a junior research associate, and Yoshihisa Harada, a visiting scientist (also a research associate professor at the School of Engineering of The University of Tokyo) of the Excitation Order Research Team, Quantum Order Research Group at RIKEN SPring-8 Center; Osamu Takahashi, an assistant professor at the Faculty of Science of Hiroshima University; Anders Nilsson, a professor at the SLAC National Accelerator Laboratory; and L. G. M. Pettersson, a professor at Stockholm University in Sweden.