Atomic nitrogen: a parameter study of a micro-scale atmospheric pressure plasma jet by means of molecular beam mass spectrometry

Absolute atomic nitrogen densities (N) in the effluent of a micro-scale atmospheric pressure plasma jet (mu-APPJ) operated in He with small admixtures of molecular nitrogen (N-2) are measured by means of molecular beam mass spectrometry. Focusing on changes of the external plasma parameters, the dependency of the atomic nitrogen density on the admixture of molecular nitrogen to the plasma, the variation of applied electrode voltage and the variation of distance between the jet nozzle and the sampling orifice of the mass spectrometer are analysed. When varying the N-2 admixture, a maximum density of atomic nitrogen of approximately 1.5 x 10(14) cm(-3) (similar to 6 ppm) is reached at about 0.25% N-2 admixture. Moreover, the N density increases approximately linearly with the applied voltage. Both results are comparable to atomic oxygen (O) behaviour of the mu-APPJ operated at equal plasma conditions except for admixing molecular O-2 instead of nitrogen (Ellerweg et al 2010 New J. Phys. 12 013021). The N density decreases continuously with increasing distance, but the decrease is slower than in the case of O atoms in He/O-2 plasma. N atoms with a density of 2.0 x 10(13) cm(-3) (similar to 0.8 ppm) are still detected at 40 mm distance from the jet nozzle in controlled He/N-2 atmosphere. The simple fluid simulation of N diffusion does not reproduce the measured densities of N. Nevertheless, a simulation taking into account atomic nitrogen reactions with gas impurities are able to reproduce the measured data, indicating that these reactions are an important loss mechanism of N atoms. The presented results are relevant for the future investigation of interactions of reactive nitrogen species with biological substrates.