Measurement of singlet delta oxygen in an atmospheric-pressure helium–oxygen plasma jet

The density of singlet delta oxygen is measured in situ in a He/O2 atmospheric-pressure plasma jet using optical emission spectroscopy at 1.27 μm. A sinusoidal high voltage of 1–10 kHz is applied between two electrodes wrapped around a quartz tube, in which the working gas flows. The two-dimensional distribution of the density shows that is primarily produced in the dielectric barrier discharge inside the quartz tube and that the production in the plasma jet plume is negligibly small. Typically, 100 ppm of is produced. The density increases with O2 concentration and is proportional to the specific input energy (SIE; J/l) irrespective of the discharge voltage, frequency, and gas flow rate. The production per SIE was 10 ppm/(J/l). The loss processes of via reactions with O3 and O2 are discussed. The density decreases when the working gas is humidified. An addition of 200 ppm of H2O decreases the density by half.