&Bullet; physics 14, p29
In water, individual electrons can clump together with water molecules to form a quasiparticle that oscillates in size, a behavior that could influence the equilibrium speed of chemical reactions in the system.
When a free electron in water interacts with neighboring water molecules, it can form a quasiparticle known as a “solvated” electron. The behavior of these solvated electrons provides fundamental insights into charge transport and chemical reactions. Michael Wörner from the Max Born Institute in Germany and colleagues have now observed that solvated electrons in water cause previously unseen oscillations in the terahertz range in the polarization of water  . These vibrations can play an important role in how a chemical reaction approaches equilibrium.
To generate solvated electrons, the researchers used pulses of terahertz and near-infrared radiation
-wide water jet. The radiation drew electrons from some water molecules. Due to interactions with other water molecules, these electrons moved a short distance before localizing. To understand the properties of the liquid, the researchers used terahertz light to monitor polarizability – which is related to how easily the liquid develops a dipole moment in response to an electric field.
The researchers observed that the polarization of the liquid oscillates on the order of magnitude. The oscillations lasted dozens of picoseconds and had a frequency between 0.2 and 1.5 THz, with the higher frequencies occurring at higher electron concentrations. Using a previously developed theoretical model, Wörner and colleagues found that this oscillatory reaction results from the quantized movement of the solvated electrons.
The researchers explained the origin of vibrations as follows: When an electron first detaches itself from its water molecule, the electron has high kinetic energy. This energy decreases when the electron interacts with other water molecules. After a certain loss of energy, the electron interacts with nearby water molecules, whose dipole moments point to the electron, and forms a solvated electron. This quasiparticle vibrates in size as it recovers the remaining energy of the electron, resulting in the terahertz vibrations measured by the team.
Sophia Chen is a freelance science writer based in Columbus, Ohio.
- A. Ghalgaoui et al., “Terahertz polaron oscillations of electrons solvated in liquid water”, Phys. Rev. Lett.126, 097401 (2021).