In an experiment at the SLAC National Accelerator Laboratory in the United States, scientists, for the first time, directly observed how excited hydrogen atoms in a water molecule interact with neighboring molecules. These quantum-mechanical interactions are the basis of hydrogen bonds, which are associated with many unique physical and chemical properties of liquid water. The results of the study are published in the journal Nature.
Water is one of the most important, but at the same time, the least studied liquids in nature. In particular, scientists do not fully understand the mechanism of the occurrence of hydrogen bonds, which largely determine the behavior of liquid water in chemical reactions.
The vibrations of atoms in water molecules have been studied by physicists in experiments using ultrafast spectroscopy. However, these experiments did not give a direct idea of the movement of atoms — scientists estimated the displacement of atomic positions in them by complex translation of spectral dynamics into the dynamics of hydrogen bonds.
Experimental physicists working at the National Accelerator Laboratory SLAC at Stanford University, together with colleagues from other universities in the United States and Sweden, used a high-speed electronic camera MeV-UED SLAC to observe the vibrations of atoms, which detects subtle movements of molecules by scattering a powerful beam of electrons from samples.
The authors created jets of liquid water with a thickness of 100 nanometers and made the water molecules vibrate using infrared laser light. Then they blew up the molecules with short pulses of high-energy electrons emitted by the camera. As a result, the scientists obtained high-resolution images of the changing atomic structure of the molecules, which they combined into a frame-by-frame film showing how a network of water molecules reacts to light.
The images, which were focused on a group of three molecules, showed that when an excited water molecule begins to vibrate, its hydrogen atom attracts oxygen atoms to neighboring molecules for a short time of about 80 femtoseconds before pushing them away with a new force, expanding the space between the molecules. At the same time, during the approach, the hydrogen bond is compressed by about 0.04 angstroms, and heating persists for one picosecond.
In other words, hydrogen atoms in water molecules continuously attract and repel neighboring molecules. Each water molecule contains one oxygen atom and two hydrogen atoms. 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.
“The small mass of hydrogen atoms enhances their quantum wave behavior,” the words of one of the authors of the study, Dr. Kelly Gaffney, a physicist from the Stanford Institute, are quoted in a press release from the SLAC National Accelerator Laboratory. — This is the first study that directly demonstrates that the reaction of a network of hydrogen bonds to an energy pulse depends on the quantum mechanical nature of how hydrogen atoms are separated and interconnected.”
The entangled quantum-mechanical network of hydrogen atoms, according to the authors, underlies many properties of water that physicists could not explain until they saw how water molecules interact with each other.