ScienceDaily maps the potential energy surfaces of water


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Liquids are harder to describe than gases, crystalline solids, or gases. A HZB team has now created a map of the potential energy surfaces for water molecules in liquid water, under ambient conditions. This was done at the Swiss Light Source SLS (Paul Scherrer Institute), Switzerland. This will allow us to better understand the chemistry and properties of water and aqueous solutions. These investigations will soon continue at the METRIXS station, which is located at the Xray source BESSY II.

Water is undoubtedly the most well-known liquid on the planet. Water is a key component in many biological and chemical processes. Water molecules don’t hold many secrets. We are already taught in school that water is composed of one oxygen atom, and two hydrogen atoms. We know what the O-H legs are made of together and the typical angle they form. Additionally, we know when water freezes or boils and how pressure affects these phase transitions. However, between the facts on individual molecules, and a deeper understanding the macroscopic phenomena there is still a lot of uncertainty. Only statistical information exists about the behaviours of individual molecules in normal liquid water. The liquid phase contains a complex network of hydrogen bonds that is disordered and dense. Their interactions are not as well understood as those in the gaseous state.

Pure liquid water examined

HZB physicist Annette Pietzsch led a team that examined pure liquid water at normal pressure and room temperature. The scientists were able to map the potential energy surfaces of individual water molecules in their ground state using X-ray analysis at Paul Scherrer Institute’s Swiss Light Source and statistical modeling. These molecules come in many shapes, depending on the environment they are in.

Measuring vibrations and oscillations

“The unique thing about this method is that we studied water molecules at the ADRESS beamline with resonant elastic X-ray scattering. Pietzsch explains that we nudged individual molecules with great care and measured their fall back to the ground state. Low-energy excitations caused stretching oscillations, and other vibrations that — when combined with model calculations — gave us a complete picture of potential surfaces.

Pietzsch explains that this gives them a way to experimentally determine the energy a molecule has as a function its structure. “The results allow us to illuminate water’s chemistry, such as how water behaves when it is a solvent.”


The HZB’s BESSY II Xray source is already planning the next experiments. Annette Pietzsch has set up the METRIXS measurement station. It is specifically designed for liquid samples and RIXS experiments. “We will begin with the first testing of our instruments after the summer shutdown for maintenance on BESSY II. Then we can continue.”


MaterialsProvided by Helmholtz-Zentrum Berlin für Materialien und Energie. Notice: Content can be edited to improve style and length.


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