mixture*

 Atmospheric-pressure cold plasma (APCP) exhibits high chemical reactivity and valuable applicability to various targets including liquids and living organisms. APCP produces high-density radicals, whereas it has the short lifetime of radicals due to frequent collisional deactivations. To address this issue, plasma generation in sub-atmospheric pressure is an effective way to extend the radical lifetime without the sacrifice of target variety. However, the behavior of radicals in sub-atmospheric pressure is still under investigation.

In this study, we measured the behavior of atomic oxygen (O) in needle-to-sphere pulsed barrier discharge in sub-atmospheric pressure He-O₂ mixture, using TALIF spectroscopy. In sub-atmospheric pressure region (50 and 70 kPa), atomic oxygen exhibits the afterglow production near the needle or temporal plateau at 50% of He fraction. The temporal plateau supposedly arises from the balance between production and decay of atomic oxygen. The production of atomic oxygen after the pulsed discharge suggest the contribution of long-lived species other than electrons, such as metastables of He. The afterglow production of atomic oxygen was more predominant near the needle anode than the barrier cathode. Spatial profiles of atomic oxygen had a relatively uniform distributions in He-O₂ mixture, whereas those in pure O₂ barrier discharge exhibited localised distribution near the barrier cathode. The atomic oxygen lifetime near the cathode was doubled by changing the gas composition from pure O₂ to He(50%)/O₂.