Velocity echoes in water provide hints about aquatic structure

Much remains unknown about water and its structure. By studying velocity echoes of water in a very low-frequency, terahertz regime, scientists can learn about its intermolecular degrees of freedom. This has been investigated experimentally in the past, and a new paper by Peter Hamm provides a computational method to reproduce and confirm interpretations of previous experimental results.

When a small disturbance is introduced in water, the size and lifetime of its echo indicate the degree of inhomogeneity and the lifetime of local intermolecular structures in the water.

“There is a lot of controversy about the structure of water and questions about the extent of its structure,” Hamm said. “The echo is so important, because it is the spectroscopic way to tackle these questions.”

Hamm simulated the molecular dynamics of a system of 5,000 water molecules. By calculating the three-point velocity correlation functions of the water, he extracted the average decay time of the echoes of the perturbation.

To confirm the relation between the echoes and the water’s structure, Hamm separately studied systems of neat water, supercooled water, amorphous ice and aqueous salt solutions. He found the more inhomogeneous the water is, the larger its echo.

“As the structures become more pronounced, the echo lifetimes go up,” he said.

Though there is a quantitative discrepancy between the results from the simulations and the experimental data, the simulations qualitatively verify experimental findings. Hamm thinks this inconsistency has to do with simplifications he made in the model in order to decrease computational costs.

“The very aspect that there is an echo is confirmed,” Hamm said.

Source: “Velocity echoes in water,” by Peter Hamm, The Journal of Chemical Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5112163 .