Since water is present in every known organism, the so-called photolysis of water is often the starting point for radiation damage. “However, the chain of reactions that can be triggered in the body by high-energy radiation is still not fully understood,” explains Inhester. “For example, even just observing the formation of individual ions and radicals in water when high-energy radiation is absorbed is already very difficult.” 

To study this sequence of events, the researchers bombarded single water molecules with pulses from the X-ray laser. These pulses are so intense, that in many cases not only one, but two or more X-ray photons were absorbed. The absorption of the second photon gave the research team a glimpse into what happens inside the molecule after the absorption of X-ray light.

“The movement of the atoms of the molecule occurring between the two absorption events leaves a clear fingerprint in the fragmentation pattern of the molecule, in other words, the fragments of the molecule fly apart in a very specific, characteristic way,” says Piancastelli. “By carefully analysing this fingerprint, as well as using detailed simulations, we were able to draw conclusions about the ultra-fast dynamics of the water molecule after it had absorbed the first X-ray photon.” This allowed the scientists to record the disintegration of the water molecule, which lasted only a few femtoseconds (quadrillionths of a second), in the form of a film.

The study shows that the disintegration of the water molecule can be much more complicated than initially expected. The water molecule (H₂O) starts to stretch and expand before eventually breaking apart. After only ten femtoseconds, the two hydrogen atoms (H), which are normally attached to the oxygen atom (O) at an angle of 104 degrees, can build up so much momentum as to face each other at an angle of around 180 degrees. As a result, the oxygen atom is not in fact flung away hard when the molecule breaks up, because the momenta of the two hydrogen nuclei largely balance each other out as they fly off, leaving the oxygen virtually at rest in the middle. In an aqueous environment, this can then easily lead to further potentially harmful chemical reactions.

“In our research, we succeeded for the first time in taking a closer look at the dynamics of a water molecule after it absorbs high-energy radiation,” says Inhester, who works at the Centre for Free-Electron Laser Science (CFEL), a collaboration between DESY, the University of Hamburg and the Max Planck Society. “In particular, we were able to characterise the formation of the oxygen and hydrogen ions and radicals more precisely, as well as the way this process unfolds over time. This disintegration of the water molecule is an important first step in the further chain of reactions that ultimately lead to radiation damage.”

The analysis adds to the overall picture of radiation effects on water. A previous study involving some members of the same team had explored the detailed dynamics of the formation of so-called free radicals by less energetic radiation in water. The processes observed there have similar dynamics to the secondary processes in the absorption of high-energy radiation now under investigation. The newly gained insights address elementary questions about reaction dynamics in water, which are to be further investigated at the Centre for Molecular Water Science (CMWS) currently being set up with international partners at DESY.

“The experiments on single water molecules were among the first we performed with the new COLTRIMS reaction microscope at SQS. These results show that we will be also able to look at other solvents and molecules with more complex structure, such as ethanol or cyclic compounds, which are of great interest in chemistry and other disciplines,” says Till Jahnke.

Involved in the current study were researchers from the universities of Frankfurt am Main, Freiburg, Hamburg and Kassel as well as Gothenburg, Lund and Uppsala in Sweden and Turku in Finland, from the Fritz Haber Institute of the Max Planck Society and the Max Planck Institute for Nuclear Physics, from Lawrence Berkeley National Laboratory and Kansas State University in the USA, the National Research Council and the Technical University of Milan in Italy, the Sorbonne in Paris, European XFEL and DESY.

Reference

Inner-Shell-Ionization-Induced Femtosecond Structural Dynamics of Water Molecules Imaged at an X-Ray Free-Electron Laser ; T. Jahnke, R. Guillemin, L. Inhester, et al.; „Physical Review X“, 2021; DOI: https://doi.org/10.1103/PhysRevX.11.041044