Water has a number of unique properties such as ice floats but a solid should be heavier than a liquid, existence of a density maximum, unusual high heat capacity and many more which forms the basis for marine and mammal life on planet Earth.

Currently, there exists no quantitative theoretical models based on simulations that can describe these anomalous properties. It was found by Stockholm researchers a few years ago that the anomalous properties of supercooled water are significantly enhance for light water in comparison to heavy water (Science 358, 1589 (2017)). All bonding properties should be identical between the two isotopes except the quantum distribution of the bond length in the waters vibrational motion.

This observation points to the importance of nuclear quantum effects also for the anomalous properties. The question is if the quantum effect could be the missing link in theoretical modelling of water to quantitatively describe the anomalous properties?

Atomic Motion in Liquid Water Molecules

Often, we think of the H2O molecule in a ball-stick model of fixed distances between the atoms. Even at low temperature there is a zero-point motion of the atoms and in a quantum description the internal OH bond length has a distribution of distances. The experiment performed by the team was to excite the molecules in water using an IR laser with a pulse duration of ≈100 femtoseconds and then use scattering of high energy electrons also with a similar pulse duration.

Since a large momentum space of the electron scattering were measured it became possible to convert the data to real space using a simple Fourier Transform technique and detect the correlations of O-O and O-H distances. The experiment was conducted such as there was a variable time delay between the IR laser and the electron beam allowing to follow time dependent distance changes in water upon the IR excitation. At t=0 the OH stretch is excited from v=0 to v=1 generating a different quantum distribution of O-H distances. Within 80 femtoseconds a transient hydrogen bond contraction of roughly 0.04 Å was detected, followed by a thermalization on a timescale of ~1 picosecond.

A contraction implies that the hydrogen bond has strengthen. At first, this observation is quite surprising since adding more energy into the OH stretch vibration may easily been assumed to lead to a bond weakening. The results demonstrate the importance of the quantum nature of the vibrational motion of the water molecules for the hydrogen bond and thereby also for the water properties, in-line with the above previous observations.

This work involved Anders Nilsson at Chemical Physics, Fysikum, Stockholm University. The study was done in cooperation with SLAC National Accelerator Laboratory, Stanford University, University of California, Davis and University of Nebraska-Lincoln all in the US.
Scientist Contact: Anders Nilsson, andersn@fysik.su.se, +46-73 9946230

More information

  • Further reading in Nature: Direct Observation of Ultrafast Hydrogen Bond Strengthening in Liquid Water by Jie Yang et al: Nature 25 August 2021 (10.1038/s41586-021-03793-9).
  • The first direct observation of atomic motion in liquid water molecules that have been excited with laser light. The results reveal the microscopic origin of water’s strangeness related to the quantum nature of the vibrational motion. (Greg Stewart/SLAC National Accelerator Laboratory)