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Potential energy surfaces of water mapped — ScienceDaily

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Liquids are more difficult to describe than gases or crystalline solids. The HZB team has now for the first time mapped the potential energy surfaces of water molecules in liquid water under ambient conditions at the Swiss Light Source SLS of the Paul Scherer Institute, Switzerland. This contributes to a better understanding of the chemical composition of water and aqueous solutions. These studies may soon be continued at the newly built METRIXS station at the BESSY II X-ray source.

Water is by far the most famous liquid in the world. Water plays a crucial role in all biological and many chemical processes. The water molecules themselves are unlikely to hide any secrets. Already at school we learn that water consists of one oxygen atom and two hydrogen atoms. We even know the typical obtuse angle that the two OH legs make with each other. Also, we know when water boils or freezes and how these phase transitions are related to pressure. But there is a large area of ​​uncertainty between facts about individual molecules and a deeper understanding of macroscopic phenomena: only statistical information is known about the behavior of individual molecules in ordinary liquid water: water molecules in the liquid phase form an oscillating shape. a network of hydrogen bonds, disordered and dense, and their interactions are not at all as well understood as in the gaseous state.

Pure liquid water is being studied

Now a team led by HZB physicist Dr. Annette Pitzsch has taken a closer look at pure liquid water at room temperature and normal pressure. Using X-ray analysis at the Swiss Light Source of the Paul Scherer Institute and statistical modeling, scientists managed to map the so-called potential energy surfaces of individual water molecules in the ground state, which come in many different forms. depending on their environment.

Measured oscillations and vibrations

“What’s special here is the method: we studied water molecules at the ADRESS beamline using resonant inelastic X-ray scattering. Simply put, we nudged individual molecules very gently and then measured how they returned to the ground state,” says Pitzsch. . The low-energy excitations led to stretching oscillations and other vibrations that, combined with model calculations, provided a detailed picture of the potential surfaces.

“This gives us a method of experimentally determining the energy of a molecule as a function of its structure,” Pitzsch explains. “The results help shed light on the chemistry of water, for example to better understand how water behaves as a solvent.”

Perspective: METRIXS on BESSY II

The next experiments are already planned at the X-ray source BESSY II at HZB. There, Annette Pitzsch and her team set up the METRIXS measurement station, specifically designed to study liquid samples with RIXS experiments. “After the summer shutdown due to BESSY II maintenance work, we will begin the first tests of our instruments. And then we can move on.”

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Materials is provided Helmholtz-Zentrum Berlin für Materialien und Energie. Note: Content can be edited for style and length.

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