Researchers observe ultrafast birth of free radicals in water

Using extremely short X-ray flashes, an international team of researchers has for the first time ever observed details of the ultrafast chemical reaction leading to the formation of aggressive radicals when water is irradiated. The study provides unprecedented insights into the process, which can lead to radiation damage in the body, but is also of considerable importance in other fields such as materials science. The researchers, led by Linda Young from Argonne National Laboratory in the USA, Zhi-Heng Loh from Nanyang University of Technology in Singapore and CFEL director Robin Santra, present their work in the current issue of Science.

The study investigated the radiolysis of water, as scientists call the splitting of a chemical compound – in this case water – by radiation. “Our body consists essentially of water,” explains Santra, who is a Lead Scientist at DESY and Professor of Physics in the Cluster of Excellence Advanced Imaging of Matter at the University of Hamburg. “We are all exposed to ionizing radiation in everyday life – whether through X-rays, natural radioactivity or, for example, cosmic radiation on air travel. Therefore, what happens here is of fundamental importance.”

In the radiolysis of water, high-energy radiation first knocks an electron out of a water molecule (H₂O), thereby ionizing it to H₂O⁺. This is followed by a so-called proton transfer, in which the ionized water molecule releases a hydrogen nucleus (proton) to an adjacent molecule. This produces an extremely reactive hydroxyl radical (OH), which can cause damage in the organism, and a hydronium ion (H₃O⁺). The process itself has been known for a long time, but how it takes place has not been observed in detail until now, partly because it is extremely fast.

“The truly exciting thing is that we’ve witnessed the very fastest chemical reaction in ionized water, which leads to the birth of the hydroxyl radical,” says Young. “The hydroxyl radical is itself of considerable importance, as it can diffuse through an organism, including our bodies, and damage virtually any macromolecule including DNA, RNA, and proteins.” A deeper understanding of radiolysis could possibly help to develop strategies to suppress the formation of the hydroxyl radical.

For the investigation, the scientists ionized water by laser and then took extremely short snapshots of the processes triggered by the ionization, using the LCLS X-ray laser of the SLAC National Accelerator Laboratory in California. The X-ray laser allows for ultra-short exposure times of only about 30 femtoseconds (millionths of a billionth of a second).

The detailed analysis of the measurement data confirms the theoretical modelling of proton transfer done by Santra's team at CFEL. “We could show that the X-ray data actually contain information on the dynamics of the water molecules that enable the proton transfer,” said Santra. “In just 50 quadrillionths of a second, the surrounding water molecules literally move in on the ionized H₂O⁺ until one of them comes close enough to grab one of its protons in a sort of handshake, turning into hydronium H₃O⁺ and leaving behind the hydroxyl radical OH.”

The work involved the Nanyang Technical University in Singapore, the Argonne National Laboratory in the USA, DESY, the University of Hamburg, the University of Uppsala (Sweden), the European XFEL X-ray laser, Northwestern University in Evanston (USA), the French research centre CNRS, the Danish Technical University in Roskilde, the University of Southern California in Los Angeles, the US research center SLAC and the University of Chicago.