The experimental set-up

Ultrafast X-ray Photon Correlation Spectroscopy at MID

Scientists were searching for ultrafast, sub-ps relaxation processes that are predicted by simulations in undercooled water and aqueous solutions. They used the split-and-delay XPCS technique to manipulate the intrinsic time structure of the machine in order to record the contrast of double-pulse scattering patterns with sub-ps pulse spacing.

Investigating hydrogen bonds in ultra-cold water

Water molecules are attracted to one another by so-called ‘hydrogen bonds’, a network of intermolecular forces that give rise to the fluidity of liquid water, as well as its sometimes-unexpected behaviour. A team of scientists conducting experiments at European XFEL’s Materials, Imaging and Dynamics (MID) instrument are probing the hydrogen bond network to understand the behaviour of water at ultra-cold temperatures. Through a better knowledge of water at these temperatures, scientists can learn about conditions that exist in the atmosphere and in extreme environments such as the freezing-cold Arctic.

The hydrogen bond acts as a bond between two molecules of water: the hydrogen atom in one molecule, and the oxygen atom in a neighbouring one. These bonds can be transient, resulting in many water molecules having a fluctuating structure, with hydrogen bonds dissolving and forming. To investigate the bonds within supercooled water, scientists split the pulses from the European XFEL at the MID instrument in two, then use the two separate pulses to take two measurements from the molecule at slightly different times.

Transient hydrogen bonds during freezing

“MID has a really special instrument which is able to split the XFEL pulses into two identical pulses with roughly half the intensity. It can then delay one pulse with respect to the other, with delays as short as a few femtoseconds to hundreds of picoseconds,” Claudia Goy, DESY explains. A femtosecond is a trillionth of a second, roughly to one second what one second is to the lifetime of the universe. “The lifetime of a hydrogen bond is around a picosecond, so this is exactly the right time range for us to look at.”

The scientists are investigating the changing network of hydrogen bonds at low temperatures, well below the regular freezing point of water. To do this, they cool water down while keeping it in its liquid form, achieved by firing a jet of water through a vacuum. As the water passes through the vacuum, evaporation happens at the droplet’s surface, causing it to cool, like sweating when you’re hot. This allows the droplet to stay liquid.

Insights with picosecond accuracy

“In our daily life, water freezes easily because it contains guests, or impurities. But for pure water, these impurities are gone, so it is possible to keep water as a liquid down to minus 35 degrees Celsius or even lower. But the catch is that it only exists in this state for a very short time, microseconds or less,” says Goy. “Ultra-cold liquid water like this displays a fascinating anomaly in terms of its density. We care about this because it behaves in ways that extend into normal temperatures, and that affects life, like fish in the deep sea.”

The measurements allow them to develop insights into the average ‘positions’ of the oxygen molecules and their velocities, on time scales at which they can make inferences about the behaviour of the hydrogen bonding network. Understanding the bonds in waters at these temperatures can give scientists an idea of how water can behave in this strange, ultracold regime, which could potentially tell us about cold water droplets in the atmosphere, like those in clouds. The researchers then hope to add impurities, such as sodium chloride, to the water to see how this impacts its behaviour.