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For the first time, an international group of experimental physicists from the United States and Sweden directly observed how hydrogen atoms in water molecules pull and push neighboring molecules when they are excited by laser light.

Each water molecule contains one oxygen atom and two hydrogen atoms, and a network of hydrogen bonds between positively charged hydrogen atoms in one molecule and negatively charged oxygen atoms in neighboring molecules holds them all together.

This intricate network is the driving force behind many unexplained properties of water, but until recently, physicists could not directly observe how a water molecule interacts with its neighbors.

"The small mass of hydrogen atoms enhances their quantum-wave behavior," the scientists say.

"This study is the first to directly demonstrate that the reaction of a network of hydrogen bonds to an energy pulse critically depends on the quantum mechanical nature of how hydrogen atoms are separated, which has long been considered responsible for the unique properties of water and its network of hydrogen bonds."

Until now, it was not easy to make this observation, because the movements of hydrogen bonds are very tiny and very fast.

In the new experiment, this problem was solved by using SLAC MeV-UED, a high-speed "electron camera" that detects subtle movements of molecules by scattering a powerful beam of electrons from samples.

The authors of the experiment created jets of liquid water with a thickness of 100 nm and made the water molecules vibrate using infrared laser light.

Then they blew up the molecules with short pulses of high-energy electrons. As a result, high-resolution images of the changing atomic structure of the molecules were obtained, which they combined into a frame-by-frame film showing how a network of water molecules reacts to light.

The images, which focused on groups of three water molecules, showed that when an excited water molecule begins to vibrate, its hydrogen atom attracts oxygen atoms closer to neighboring water molecules before pushing them away with its newfound strength, expanding the space between the molecules.

"Although it was assumed that the so – called nuclear quantum effect underlies many of the strange properties of water, this experiment is the first time that it has been observed directly," said Professor Anders Nilsson, a researcher at Stockholm University.

"For a long time, researchers have been trying to understand the network of hydrogen bonds using spectroscopy methods. The beauty of this experiment is that for the first time we were able to directly observe how these molecules move."

The results are published in the journal Nature

PHOTO by Greg Stewart / SLAC National Accelerator Laboratory.

Author: Arseniy Shchukin

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