Liquids are more difficult to describe than gases or crystalline solids. An HZB team has for the first time mapped the potential energy surfaces of water molecules into liquid water under ambient conditions at the Swiss Light Source SLS at the Paul Scherrer Institute, Switzerland. This contributes to a better understanding of water chemistry and aqueous solutions. These investigations will soon be able to continue at the newly built METRIXS station at the BESSY II X-ray source.
Water is without a doubt the best known liquid in the world. Water plays a crucial role in all biological processes and many chemical processes. The water molecules themselves keep almost no secrets. At school we already learn that water consists of one oxygen atom and two hydrogen atoms. We even know the typical obtuse angle that the two OH legs form with each other. In addition, we know when water boils or freezes and how these phase transitions relate to pressure. But between the facts about individual molecules and a deeper understanding of macroscopic phenomena, there is a wide area of uncertainty: only statistical information is known about the behavior of individual molecules in normal liquid water: water molecules in the liquid phase forms a fluctuating structure. hydrogen bonding network, disordered and dense, and their interactions are not understood at all as well as in the gaseous state.
Pure liquid water examined
Now, a team led by HZB Physics, Dr. Annette Pietzsch, has taken a closer look at pure liquid water at room temperature and normal pressure. Through X-ray analysis at the Swiss light source of the Paul Scherrer Institute and statistical modeling, scientists have been able to map the so-called potential energy surfaces of individual water molecules in the ground state, which they come in a variety of shapes. according to their environment.
Measured oscillations and vibrations
“The special thing here is the method: we studied the water molecules in the ADRESS light line using resonant and inelastic X-ray scattering. Simply put, we moved individual molecules very carefully and then measured how they returned to fall into the ground state, ”says Pietzsch. . Low-energy excitations caused stretching oscillations and other vibrations, which, combined with the model’s calculations, produced a detailed picture of the potential surfaces.
“This provides us with a method for experimentally determining the energy of a molecule based on its structure,” Pietzsch explains. “The results help illustrate the chemistry of water, for example to better understand how water behaves as a solvent.”
Perspectives: METRIXS to BESSY II
Upcoming experiments are already planned at the BESSY II X-ray source at HZB. There, Annette Pietzsch and her team have installed the METRIXS measuring station, which is designed precisely to investigate liquid samples with RIXS experiments. “After the summer break for maintenance work on BESSY II, we’ll start with the first tests of our instruments. And then we can move on.”
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