First time mapping of the potential energy surfaces for water

Water is the most well-known liquid in this world.

First time mapping of the potential energy surfaces for water

Water is the most well-known liquid in this world. It is a vital component of many biological and chemical processes. Water molecules are not secretive. We learn in school that water is composed of one oxygen atom, and two hydrogen atoms. We also know the typical angle at which the O-H legs meet. We also know when water boils and freezes, as well as how phase transitions relate to pressure.

There is still much to be known, both about individual molecules and the macro-scale phenomena. The liquid phase of water contains a complex network of hydrogen bonds. Their interactions are less well understood than 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.

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. The low-energy excitations led to stretching oscillations and other vibrations, which--combined with model calculations--produced a detailed picture of the 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 can be used to illuminate the chemistry of water, such as to better understand how water behaves in its role as a solvent.

These results were published in Proceedings of the National Academy of Sciences.


At the HZB, the next experiments have already been planned for the BESSY IIX-ray source. Annette Pietzsch has set up the METRIXS measurement station. It is specifically designed for liquid samples and RIXS experiments. "After the summer shutdown for maintenance on BESSY II we will begin with the first tests. Then we can continue."