Femtosecond X-ray experiment research on structural dynamics of water at FXE
Scientists have used FXE to study hydrogen bond breaking and structural dynamics of the water tetrahedral cage that accommodates an interstitial water molecule after IR laser excitation.
Observing ultrafast processes
It may be considered a given that we know all we need about water’s structure. It has, after all, a simple chemical formula: H2O, or two hydrogen atoms bonded together with one oxygen atom. Despite its simple formula, there is still much to be discovered about the structure and behaviour of water. One such area of investigation being undertaken at European XFEL is how the structure of liquid water changes under different conditions, such as under illumination from intense radiation or at high temperatures. These studies of water’s so-called ‘structural dynamics’ can potentially inform scientists about previously unknown phenomena, giving better insights into one of the most important substances on Earth.
“If we want to understand the mechanics of a material, we need to understand its structural properties,” says Qingyu Kong, academic at the SOLEIL synchrotron in France, and leader of the study conducted at European XFEL. “This is also true for water. If its structural properties change with time, and at different temperatures, understanding these changes can help us learn about water in its natural environment.”
To investigate how water’s structure changes while it is heated, scientists can use a combination of infrared radiation (heat) produced by a laser, and the ultrashort X-ray flashes produced by the European XFEL, which are just trillionths of a second (femtoseconds) long. By illuminating water molecules with infrared light at 2000 nm, researchers induce heating. They can then examine the way water changes during heating by ‘probing’ the hot molecules using the ultrashort X-ray flashes. The measurements tell them about how the bonds between the atoms in water change in length, which enables clearer insights into how water molecules move and vibrate under heating. Femtosecond X-ray pulses can also provide scientists with critical information about how bonds form and break during chemical reactions and other processes.
To be able to make such measurements, researchers require an X-ray “camera” that can take images extremely quickly. One such unique “camera” is the pump-probe technique implemented at the European XFEL’s Femtosecond X-ray Experiments (FXE) instrument. FXE’s capabilities allow scientists to measure processes that are shorter than 100 fs long.
Heating water
“To probe water’s structural dynamics, you need both the high intensity X-ray and also the high speed,” Kong continues. “FXE is a really special instrument. Its extremely high-speed X-ray “camera” allows us to see the motion of hydrogen and oxygen atoms in real time.”
Learning more about water and how its structure transforms can tell scientists about how the molecule behaves, which can in turn lead to more effective technology that utilises water, for example, in biological or natural systems. Crucially, learning about how liquid water can be heated in its natural environment can tell us about the ways in which our oceans are heating due to climate change. Such knowledge could potentially help us find better outcomes in the search to reduce the impacts of global warming.