29-Jun-2018 - Universität Zürich

THz spectroscopy could help Explain water's anomalies

Liquid water sustains life on earth, but its physical properties remain mysterious among scientific researchers. Recently, a team of Swiss researchers used existing THz spectroscopy techniques to measure liquid water's hydrogen bonding. Future efforts with this technique could one day help explain water's peculiar properties.

"What makes this paper special is the aspect of THz spectroscopy of a liquid. In the THz range, we make spectroscopy explicitly of the intermolecular degrees of freedom of the system in the study, to contrast it to intramolecular degrees of freedom," said Peter Hamm, an author on the paper. "With THz spectroscopy, we can very directly look at the hydrogen bonding between various water molecules."

Researchers use spectroscopy to measure the interaction of matter with light and infer something's physical composition. In this experiment, the researchers excited a dye molecule dissolved in water with an ultrashort visible laser pulse, changing its charge distribution. Then, a THz pulse measured the response of the surrounding water molecules as a function of the time after that excitation process.

THz spectroscopy, which is relatively low frequency, enables researchers to examine the forces that exist between water molecules. Observing these intermolecular forces could help researchers understand water's anomalies, because hydrogen bonding in liquid water molecules make up many of water's unexpected properties, like its unusual density maximum at 4 degrees Celsius.

"The response we found in the THz frequency range was surprisingly slow. Water is typically considered to be a very fast solvent with a response in the subpicosecond range, but we found a timescale around 10 picoseconds in the THz," Hamm said. They attributed the slow timescale to the collective nature of the water response that was probed using THz spectroscopy.

Hamm clarified that researchers have been using THz spectroscopy for more than 20 years and cautioned optimism about the results. "The outcome often has been a bit disappointing because the THz spectra of liquids like water are extremely broad and blurred, and it's very hard to extract information out of that," he said. Time-resolved techniques, like in this study, might overcome that limitation.

Next, the researchers plan to use their method to look at water's structure and dynamics when it's still liquid, but below the freezing point. Hamm explained, "The special properties of water become significantly more pronounced if one goes to temperatures below the freezing point."

Facts, background information, dossiers
  • terahertz spectroscopy
  • Lennard-Jones spheres
More about Universität Zürich
  • News

    Machine learning cracks quantum chemistry conundrum

    A new machine learning tool can calculate the energy required to make -- or break -- a molecule with higher accuracy than conventional methods. While the tool can currently only handle simple molecules, it paves the way for future insights in quantum chemistry. "Using machine learning to so ... more

    Trust in Science and Research Remains High

    The Swiss population’s trust in science and research is high to very high. As the Science Barometer Switzerland 2019 study shows, people in Switzerland have a positive attitude towards science and are keen to receive information about research, with climate and energy considered the most im ... more

    Recycling carbon dioxide from the ocean

    Paper, tin cans, glass - the world recycles as much as possible. So why not declare the greenhouse gas carbon dioxide (CO2) a recycling product as well? Liquid fuels based on carbon will continue to play an important role in the future - despite international efforts to reduce them. So it s ... more

More about American Institute of Physics
  • News

    Solar cell keeps working long after sun sets

    About 750 million people in the world do not have access to electricity at night. Solar cells provide power during the day, but saving energy for later use requires substantial battery storage. In Applied Physics Letters, by AIP Publishing, researchers from Stanford University constructed a ... more

    Turning plastic grocery bags into sustainable fuel

    More than 300 million tons of plastic waste are produced annually, which causes serious environmental issues because of plastic's life cycle and the difficulty of eliminating it. Consequently, most plastic waste ends up in either a landfill or the ocean. A significant number of plastics bre ... more

    Turning hazelnut shells into potential renewable energy source

    Biomass is attracting growing interest from researchers as a source of renewable, sustainable, and clean energy. It can be converted into bio-oil by thermochemical methods, such as gasification, liquefaction, and pyrolysis, and used to produce fuels, chemicals, and biomaterials. In Journal ... more