A spectroscopic technique that identifies water molecules on the surface shows how they relax after being excited.

In a study published in the journal RIKEN scientists, a more complete picture of how excited water molecules lose energy at the interface with air has been discovered. Nature communication. This finding will be valuable for a better understanding of the processes that occur in water surfaces.

Water is an anomaly in many ways. For example, its freezing and boiling points are much higher than expected, and its density as a solid (ice) is lower than as a liquid.

Almost all of the unusual properties of water arise from the weak bonds that are constantly being formed and broken between neighboring water molecules. These bonds, known as hydrogen bonds, are formed because oxygen attracts more electrons than hydrogen. Therefore, slightly negative oxygen in one molecule is attracted to slightly positive hydrogens in other molecules.

But a small fraction of water molecules—those on the surface—experience hydrogen bonds differently than other water molecules. In their case, the arm attached to air does not form a hydrogen bond.

Until now, no one has been able to figure out how the arms of these surface molecules relax after being stretched. Because it is very challenging to isolate the signal from these molecules.

“We have a good knowledge of how water molecules behave in a liquid body, but our understanding of water molecules at the interface is lagging behind,” says Tahi Tahara of the RIKEN Molecular Spectroscopy Laboratory.

For the past decade, a team led by Tahara has been trying to remedy this situation by developing highly sophisticated spectroscopic techniques to probe the interactions of water molecules at surfaces.

The team has now developed a technique based on infrared spectroscopy that is sensitive enough to detect how the oxygen-hydrogen bonds of surface water molecules loosen.

Using this technique, the team found that oxygen-hydrogen bonds that stick in air first rotate without losing energy. Then they loosen in a similar way to the molecules in the liquid body that form the hydrogen bond network.

“In this sense, there is not much difference between molecules at the interface and inside the liquid after interacting with their neighbors—both have the same relaxation process,” Tahara says. These findings paint a comprehensive picture of how the tension of oxygen-hydrogen bonds relaxes at the surface of water.

Tahara and his team now plan to use their spectroscopic technique to investigate the chemical reactions that occur at the water interface.