Scientists manage to 'see' oxygen atoms in water for the first time

For the first time, scientists have directly seen atomic oxygen, which are single oxygen atoms not bound into molecules, moving through liquid water.

These atoms stick around for tens of microseconds, a few millionths of a second, which is far longer than chemists expected.

In a new study, a team used an ultrafast laser to capture faint flashes of light from oxygen atoms dissolved in water. 

Their measurements show that the atoms can slip hundreds of micrometers below the surface, roughly a few thousandths of an inch.

Importance of single oxygen atoms

The work was led by Brayden Myers, a postdoctoral scientist at the Swiss Plasma Center at the École Polytechnique Fédérale de Lausanne (EPFL). 

His research focuses on plasmas that generate reactive oxygen species in liquids for medical and industrial applications.

Chemists use the term reactive oxygen species, highly energetic forms of oxygen that can swiftly change nearby molecules. 

One recent review shows that these species can harm cells at high levels yet help regulate normal physiology.

Atomic oxygen sits at the extreme end of that family, because a single unpaired electron makes it a very strong oxidizer. 

In liquids, controlling where and how long it exists could improve surface sterilization and new forms of plasma treated water for healthcare.

Catching atoms with ultrafast light

To create streams of atomic oxygen for the experiment, the team used plasma, a gas where many of the atoms are electrically charged. 

The microplasma jet blew a helium and oxygen mix across a small pool of deionized water, pushing fresh oxygen atoms into the liquid.

Detecting those atoms inside water is hard, because the liquid cancels excited states almost instantly and many chemical probes break down. 

The researchers solved this by switching to a femtosecond, one million-billionth of a second, laser pulse that packs light into a short burst.

They then used two-photon absorption laser-induced fluorescence, a laser method that excites atoms and records their glow, to watch the oxygen atoms. 

Two photons from the ultraviolet pulse lifted each atom to a higher energy state, then the atom emitted a tiny flash the camera recorded.

What the scientists saw

“This observed longevity has significant implications, suggesting a need to re-evaluate existing models of solvated atomic oxygen reactivity and transport,” wrote Myers. 

The images showed oxygen atoms glowing, not only at the surface, but deeper in the water. That survival time is long enough for atoms to be carried downward by the small flow where the gas jet hits the water. 

When the team calibrated the fluorescence, they found concentrations near the surface of about ten quadrillion oxygen atoms in each cubic centimeter of water.

Careful analysis of the chemical pathways suggests that these oxygen atoms rarely attack water itself and mostly react with dissolved oxygen to form ozone. 

By matching gas and liquid measurements, the team could estimate a Henry’s law constant, a number that links gas concentrations to dissolved amounts.

Changing plasma chemistry in water

Cold atmospheric plasma jets are already being explored for sterilizing medical tools, helping wounds heal, processing food, and treating contaminated water. 

A recent perspective on plasma liquid interactions argues that knowing which species reach the liquid at what doses is essential for treatment design.

Because the atoms last and travel farther in water, plasma based treatments might influence deeper layers of tissue or biofilms than some models assume. 

That could help when the goal is to inactivate microbes in crevices, but it also means dose limits for healthy cells need revision.

The imaging method is not limited to oxygen, because it could be tuned to look at nitrogen or hydrogen atoms in liquids. 

Watching where those atoms go over microsecond timescales would help scientists map how plasmas or electric fields reshape chemistry inside small volumes of water.

Looking ahead to new questions

Researchers now want to know how atomic oxygen behaves in natural water that contains salts, organic molecules, and living cells. Those factors could shorten its lifetime or redirect its reactions in unexpected ways.

Teams are also exploring whether the same laser approach can map other reactive atoms under more complex conditions. Early tests suggest the method can be adapted, but the detection limits will need improvement.

Oxygen atoms and plasma

For now, the main achievement is a set of direct pictures and measurements that pin down how atomic oxygen behaves in simple water. 

With lifetimes, transport distances, and approximate concentrations in hand, researchers can build more reliable models that connect atomic scale events with macroscopic effects.

Those models will matter across disciplines, from designing disinfecting plasma treated water to shaping reactors that use oxygen atoms to clean or modify surfaces. 

Being able to see these atoms in water connects an invisible part of chemistry to practical decisions about safety and plasma driven technologies.

The study is published in Nature Communications.

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