Determination of atomic oxygen in atmospheric plasma from oxygen isotope exchange

Oxygen scrambling is observed when a 1:1 mixture of (16)O(2) and (18)O(2) in nitrogen/helium buffer (total O(2): 5-20%) is treated at atmospheric pressure in a surface discharge plasma reactor. The statistical isotope distribution ((16)O(2)/(18)O(2)/(16)O(18O) = 1:1:2) is achieved with a very low SIE (i.e., the energy per unit volume). This process, implemented by kinetic simulations, is proposed as a chemical probe for the determination of the average O-atom density in atmospheric plasmas. It was found that, within the explored composition range (5-20% O(2) in N(2)/He buffer), the average O-atom density increases with SIE, while it decreases with increasing content of O(2) in the gas being treated. In contrast, an opposite trend is observed for ozone production, which increases with the concentration of O(2) in the treated gas. These observations are consistent with the known reaction of O-atoms with O(2) to form ozone, which, at constant total pressure, is expected to favor ozone production as the concentration of O(2) is increased.