The mystery of O and O3 production in the effluent of a He/O2 atmospheric pressure microplasma jet

TL;DR

This study investigates the production and spatial distribution of atomic oxygen and ozone in the effluent of a He/O2 microplasma jet, revealing unexpected ozone behavior in ambient air with implications for plasma medicine.

Contribution

It combines experimental measurements and fluid modeling to explain the behavior of reactive oxygen species in plasma jet effluents under different atmospheric conditions.

Findings

Oxygen atom density decreases slightly faster in air than in helium.

Ozone density increases significantly faster in air with distance.

The proposed reaction scheme explains the observed densities accurately.

Abstract

Microplasma jets are commonly used to treat samples in ambient air atmosphere. The effect of admixing air into the effluent may severely affect the composition of the emerging species. Here, the effluent of a He/O2 microplasma jet has been analyzed in a helium and in an air atmosphere by molecular beam mass spectrometry. First, the composition of the effluent in air has been recorded as a function of the distance to determine how fast air admixes into the effluent. Then, the spatial distribution of atomic oxygen and ozone in the effluent has been recorded in ambient air and compared to measurements in a helium atmosphere. Additionally, a fluid model of the gas flow with reaction kinetics of reactive oxygen species in the effluent has been constructed. In ambient air, the O density declines only slightly faster with the distance compared to a helium atmosphere. On the contrary, the O3 density in ambient air increases significantly faster with the distance compared to a helium atmosphere. This mysterious behavior can have big implication for the use of similar jets in plasma medicine. It is shown that photodissociation of O2 and O3 is not responsible for the observed effect. A reaction scheme involving the reaction of plasma produced highly vibrationally excited O2 with ground state O2 molecules is proposed as a possible explanation of the observed densities. A very good agreement between measured and simulated densities is achieved.