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By combining persistence with advanced spectroscopy, Dr. Woongmo Sung has helped answer a decades-old question about one of nature’s most familiar interfaces. 

When a Simple Surface Reveals Hidden Complexity

Water is essential to life, yet the molecular structure of its surface remains surprisingly difficult to understand. The boundary where water meets air is involved in atmospheric chemistry, environmental processes, and many biological and chemical reactions. Despite its importance, the behavior of the outermost layer of water molecules has puzzled scientists for decades.

For Dr. Woongmo Sung, this unanswered question became the starting point for a long-term research challenge.

“The air–water interface appears to be a very simple system, but it is remarkably complex. It plays an important role in many chemical and physical processes, so understanding its molecular-level structure gives us insight into a wide range of interfacial phenomena.”

One of the longest-standing questions concerns the free OH group at the water surface. Scientists have observed two spectral features in this region for many years, but their origin has remained controversial.

To address this problem, Sung used two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy, an advanced technique capable of probing only the outermost layer of water molecules.

The study showed that the two spectral features originate from the same surface water molecules, resolving a long-standing debate and providing a clearer picture of the molecular structure of the air–water interface.

Years of Preparation for a Weak Signal

Obtaining the answer was far from straightforward.

Because the experiment measures only the topmost molecular layer—less than a nanometer thick—the signal is extremely weak. The water surface and laser system had to remain stable throughout experiments that often continued for several days.

“Our spectroscopy probes only the topmost molecular layer of water, so the signal is much weaker than in conventional bulk measurements. I spent several years improving the experimental setup and developing a more robust measurement protocol before we were finally able to obtain reliable 2D spectra.”

The work also demanded persistence. During the COVID-19 pandemic, Sung spent nearly two years overcoming technical problems before collecting the first reliable spectra. Discussions with Chief Scientist Tahara led to improvements in the experimental design, eventually making high-quality measurements possible.

An Unexpected Discovery

The original goal was to determine the relationship between the two free OH bands. Instead, the data revealed something entirely unexpected.

“I also discovered something completely unexpected—a diagonal elongation of the main free OH peak.”

Further analysis showed that the outermost water molecules experience different local hydrogen-bonding environments. This finding went beyond the team’s original objective and opened a new perspective on the microscopic structure of interfacial water.

“This was a completely new finding that went beyond our original goal, and it was probably the most exciting moment of the entire project.”

Learning at PRI

Sung joined the RIKEN Pioneering Research Institute (PRI) to work in Tahara Group, Molecular Spectroscopy Laboratory, internationally recognized for its pioneering work in ultrafast nonlinear spectroscopy.

“I joined the group to learn state-of-the-art ultrafast spectroscopy from one of the best teams in the world.”

During the past nine years, he has developed advanced experimental techniques while working alongside researchers with diverse expertise.

“I have learned an enormous amount from Dr. Tahara and my colleagues.”

Advice for Young Researchers

Reflecting on his experience in Japan, Sung emphasizes that research is shaped not only by experiments but also by the people around you.

“At first, I was quite shy and found it difficult to communicate with others. However, my Japanese colleagues were always willing to help, whether with scientific problems or everyday life outside the laboratory.”

His advice to young researchers is simple.

“Keep an open mind and actively communicate with people around you. Those interactions will broaden your perspective and deepen your understanding of science.”

Looking to the Future

Although this study answers one important question about interfacial water, Sung is already looking beyond it.

“I would like to continue developing advanced nonlinear spectroscopic techniques.”

His next goal is to observe reaction intermediates and reaction dynamics directly at interfaces and, ultimately, to develop imaging methods that reveal molecular dynamics with both high spatial and temporal resolution.

Looking further ahead, he hopes to share the expertise he gained at PRI with the next generation of researchers.

“I hope to bring the techniques and research philosophy that I learned in Dr. Tahara's laboratory to Korea and help build an international research network connecting scientists across countries. That is my long-term dream.”

Woongmo Sung, Satoshi Nihonyanagi, and Tahei Tahara Journal of the American Chemical Society 2026 148 (17), 18074-18079 DOI: 10.1021/jacs.6c02191

Molecular Spectroscopy Laboratory: https://www2.riken.jp/lab/spectroscopy/tahara-e.html