Production and loss of O₂(¹Δ ) at atmospheric pressure using microwave-driven microplasmas

We have used arrays of microwave-generated microplasmas operating at atmospheric pressure to generate high concentrations of singlet molecular oxygen, O₂(¹Δ ), which is of interest for biomedical applications. The discharge is sustained by a pair of microstrip-based microwave resonator arrays which force helium/oxygen gas mixtures through a narrow plasma channel. We have demonstrated the efficacy of both NO and less-hazardous N₂O additives for suppression of ozone and associated enhancement of the O₂(¹Δ ) yield. Quenching of O₂(¹Δ ) by ozone is sufficiently suppressed such that quenching by ground state molecular oxygen becomes the dominant loss mechanism in the post-discharge outflow. We verified the absence of other significant gas-phase quenching mechanisms by measuring the O₂(¹Δ ) decay along a quartz flow tube. These measurements indicated a first-order rate constant of (1.2 ± 0.3) × 10^-24 m³ s^-1, slightly slower than but consistent with prior measurements of singlet oxygen quenching on ground state oxygen. The discharge-initiated reaction mechanisms and data analysis are discussed in terms of a chemical kinetics model of the system.