Oxygen Plasma Works Better When You Take Air Away. The Missing Radicals Were Coming From the Part That Doesn't Glow.

Plasma is the fourth state of matter, after solid, liquid and gas: a soup of charged atoms and free electrons that makes up an estimated 99.9% of the matter in the universe and almost none of the matter on Earth, where it turns up mainly inside lightning. Run a high voltage across a gas and you can manufacture it — a so-called nonthermal plasma, in which the electrons are driven to enormous temperatures while the atoms stay cool and become chemically ferocious.

Industry uses that ferocity constantly. Nonthermal oxygen plasma kills microorganisms, chemically modifies surfaces, breaks down contaminants, and conditions wafers in semiconductor manufacturing. The workhorse is atomic oxygen — single oxygen atoms, split out of O2, that will oxidize nearly anything they touch. The problem has always been that they do not last. At atmospheric pressure the atoms collide with surrounding gas molecules so often that most of them are gone before they reach whatever the operator wanted treated.

A team led by Associate Professor Yusuke Nakagawa at Tokyo Metropolitan University went looking for where in a discharge those atoms are actually made. Working with pulsed electrical discharges through pure oxygen gas at pressures slightly below atmospheric, the researchers used lasers to make atomic oxygen fluoresce at a specific wavelength, then mapped how much of it appeared at each point around the positive and negative electrodes. The work is published in the Journal of Physics D: Applied Physics.

The expected trade-off did not appear. Dropping the pressure lengthened the radicals' lifetime, as anyone would predict, but the number of radicals produced stayed comparable to what the same discharge made at full atmospheric pressure. Longer life at no cost in yield meant several times more atomic oxygen was still available to react by the time it reached a target.

The reason showed up in the map. Atomic oxygen was not being produced only in the visibly glowing regions of the plasma. A substantial amount of it was coming from the dark zones close to the electrodes, where the light output is negligible. That implicates electrons of moderate energy — not just the high-energy population responsible for the glow — in splitting oxygen molecules apart, which is both a new route for radical production and the explanation for why the yield holds up when the pressure comes down.

The practical version is short: run oxygen plasma somewhat below atmospheric pressure and you get more useful chemistry for the same electrical input. For biomedical sterilization, agriculture and semiconductor surface treatment, that is a straight efficiency gain, and it comes from noticing that the interesting part of a plasma is not necessarily the part you can see.