Press Release: Ionisation of Water; an Ultrafast Story on How Radicals are Formed


In an international collaboration between research groups in Sweden, USA, Singapore, Germany, France and Denmark a new step has been taken towards the understanding of ultrafast chemical processes. In measurements at the free-electron laser in Stanford, the Linac Coherent Light Source (LCLS), they have studied how the molecules behave in water just after ionization.

X-rays capture the ultrafast proton transmission reaction in ionised water and give rise to the hydroxyl radical (OH) and the hydronium ion (H3O+).
Photo/image: Argonne National Laboratory.

“It is fantastic that one actually can follow processes that fast and in addition in such a complicated system as water. We saw early that hydroxyl radicals were formed after the ionisation, but I had hardly dared to hope that we would see how it happens”, says Ludvig Kjellsson, PhD at the Department of Physics and Astronomy at Uppsala University and one of the co-authors of the study.

What happens when water is ionised is important within everything from basic molecular physics, to chemistry and biology. Understanding the processes triggered when water is ionised also has a direct practical significance for technical applications where ionising radiation occur. The cooling water in our nuclear reactors is exposed to ionising radiation, which contributes to corrosion of the materials the water comes in contact with. Living organisms are mostly made of water, and during medical treatments and diagnostics with ionising radiation it is mainly water that is ionised. The study also shows what happens at the atomic level when we are exposed to ionising radiation.

The researchers have used the short intensive x-ray pulses created by the free-electron laser LCLS in Stanford to get a clear picture of the fast processes. After a few femtoseconds the vacancy in the electron shell created at the ionization is localised. There arises a positively charged H2O+ ion so short lived, that it has not been detected until now. In this study the researchers can for the first time follow the process when this molecular ion emits one of its protons to a neighbouring molecule so that a H3O+ ion and an OH radical is formed. The measurements show that the process takes about 46 femtoseconds, and thereafter one sees how the energy of the nuclear motions of the OH radical is spreading in the water, and finally one of the released electrons from the ionization process is captured so that an OH- ion is formed.

Through a deeper mechanistic understanding of how the chemically aggressive hydroxyl radical is formed during radiolysis of water, it might eventually be possible to develop strategies to suppress the process and thereby avoid radiation damage.

Studies of this type are always the result of collaborations involving a large number of research groups. The work has been led by Linda Young at Argonne National Laboratory in Chicago.

“Linda is one of the pioneers when it comes to experiments with molecules at free-electron laser facilities, and she has an excellent ability to organise research expeditions of this kind, bringing together experts from different fields. She contacted me to discuss what one may do with x-ray spectroscopy, which is my specialty”, says Jan-Erik Rubensson, Professor of physics at Uppsala University.

The collaboration has gathered researchers with different key competences: Zhi-Heng Loh from NTU Singapore is a specialist in time-resolved studies and has long studied fast processes in liquids. Bill Schlotter at SLAC has been indispensable with his energy, enthusiasm and his deep knowledge of the instruments at LCLS. For the interpretation of the experimental results, advanced theories have been developed by Robin Santra at CFEL in Hamburg, who is the leading theorist within the field.


Ludvig Kjellsson, PhD at the Department of Physics and Astronomy at Uppsala University

Article reference:

Z.-H. Loh1 et al; Observation of the fastest chemical processes in the radiolysis of water. Science 10 Jan 2020: Vol. 367, Issue 6474, pp. 179-182.

Elin Bäckström

Read more about the research:

Scientists observe ultrafast birth of radicals press release about the research (Argonne National Laboratory).

Researchers observe ultrafast birth of free radicals in water article about the research (Deutsches Elektronen-Synchrotron DESY).

Translation: Johan Wall