Absolute atomic oxygen and nitrogen densities in radio-frequency driven atmospheric pressure cold plasmas: Synchrotron vacuum ultra-violet high-resolution Fourier-transform absorption measurements

Reactive atomic species play a key role in emerging cold atmospheric pressure plasma applications, in particular, in plasma medicine. Absolute densities of atomic oxygen and atomic nitrogen were measured in a radio-frequency driven non-equilibrium plasma operated at atmospheric pressure using vacuum ultra-violet (VUV) absorption spectroscopy. The experiment was conducted on the DESIRS synchrotron beamline using a unique VUV Fourier-transform spectrometer. Measurements were carried out in plasmas operated in helium with air-like $N2/O2$ (4:1) admixtures. A maximum in the O-atom concentration of $(9.1 ± 0.7)×1020 m−3$ was found at admixtures of 0.35 vol. %, while the N-atom concentration exhibits a maximum of $(5.7 ± 0.4)×1019 m−3$ at 0.1 vol. %.