Water molecules are kept together by the so-called hydrogen bond, an electrostatic force between such molecules. It is of incredible importance also in proteins and DNA. And it gives water some of its peculiar properties. And now researchers have directly observed how it can tug molecules when one is excited by a laser.

The study is published in Nature. Water molecules are made of two hydrogen atoms and one oxygen atom connected by covalent bonds. The hydrogen bond is between the hydrogen in one molecule (which is slightly positive) and the oxygen in another (which is slightly negative).

To study it, researchers gave the covalent bond of a molecule a bit more energy. This led to stronger vibrations between the oxygen and the hydrogen, and in turn, this affected the water molecules around it. They were tugged in and pushed away with more strength following the excitation.  

“For a long time, researchers have been trying to understand the hydrogen bond network using spectroscopy techniques,” lead author Professor Jie Yang, from Tsinghua University in China, said in a statement. “The beauty of this experiment is that for the first time we were able to directly observe how these molecules move.”

The hydrogen bond is believed to be the reason why water boils at such a high temperature, how its crystals are shaped when it turns to ice (including snowflakes), and also why ice is less dense than water at the same temperature. Understanding the quantum nature of this force might reveal much more about this common liquid.

“Although this so-called nuclear quantum effect has been hypothesized to be at the heart of many of water’s strange properties, this experiment marks the first time it was ever observed directly,” added co-author Professor Anders Nilsson, from Stockholm University. “The question is if this quantum effect could be the missing link in theoretical models describing the anomalous properties of water.”

The experiment required incredible precision to be possible. The researchers measured a contraction of 4 picometers (4×10^-12 meters) in the bond. That’s about one-twelfth of the diameter of a hydrogen atom. And this tug lasted for 0.08 picoseconds. Incredibly quick.

“The low mass of the hydrogen atoms accentuates their quantum wave-like behavior,” explained Dr Kelly Gaffney, a scientist at the Stanford Pulse Institute at SLAC. “This study is the first to directly demonstrate that the response of the hydrogen bond network to an impulse of energy depends critically on the quantum mechanical nature of how the hydrogen atoms are spaced out, which has long been suggested to be responsible for the unique attributes of water and its hydrogen bond network.”

The work is not just a new window into understanding water, but by working out how the hydrogen bond happens and evolves we can gather a better grasp of many other chemical and biological processes.