Spatially and temporally resolved measurements of atomic hydrogen concentration above the dielectric of coplanar barrier discharge are presented for atmospheric pressure in 2.2% H 2 /Ar. The measurements were carried out in the afterglow phase by means of two-photon absorption laser-induced fluorescence (TALIF). The difficulties of employing the TALIF technique in close proximity to the dielectric surface wall were successfully addressed by taking measurements on a suitable convexly curved dielectric barrier, and by proper mathematical treatment of parasitic signals from laser-surface interactions. It was found that the maximum atomic hydrogen concentration is situated closest to the dielectric wall from which it gradually decays. The maximum absolute concentration was more than 10 22 m −3 . In the afterglow phase, the concentration of atomic hydrogen above the dielectric surface stays constant for a considerable time (10 μs-1 ms), with longer times for areas situated farther from the dielectric surface. The existence of such a temporal plateau was explained by the presented 1D model: the recombination losses of atomic hydrogen farther from the dielectric surface are compensated by the diffusion of atomic hydrogen from regions close to the dielectric surface. The fact that a temporal plateau exists even closest to the dielectric surface suggests that the dielectric surface acts as a source of atomic hydrogen in the afterglow phase.

No abstract

Concentration of atomic oxygen was measured in a volume dielectric barrier discharge (DBD) ignited in mixtures of Ar + O 2 (+ H 2 ) at atmospheric pressure. Two-photon absorption laser induced fluorescence (TALIF) of atomic oxygen was used and this method was calibrated by TALIF of Xe in a mixture of argon and a trace of xenon. The calibration was performed at atmospheric pressure and it was shown that quenching by three-body collisions has negligible effect on the life time of excited Xe atoms. The concentration of atomic oxygen in the DBD was around 10 21 m −3 and it was stable during the whole discharge period. The concentration did not depend much on the electric power delivered to the discharge provided that the power was sufficiently high so that the visible discharge filled the whole reactor volume. Both the addition of hydrogen or replacing of argon by helium led to a significant decrease of atomic oxygen concentration. The TALIF measurements of O concentration levels in the DBD plasma performed in this work are made use of e.g. in the field analytical chemistry. The results contribute to understanding the processes of analyte hydride preconcentration and subsequent atomization in the field of trace element analysis where DBD plasma atomizers are employed.

No abstract

We investigate the electric field development in weak microseconds-lasting atmospheric pressure Townsend discharge (APTD) operated in barrier discharge arrangement in pure nitrogen. The electric field is determined using four different methods: laser-aided EFISH (electric field induced second harmonics), optical emission- based FNS/SPS (first negative/second positive systems of molecular nitrogen) intensity ratio, electrical equivalent circuit approach and via determination of the Townsend first coefficient α(E/N) from the optical emission profile. The resulting values of the electric field obtained by the respective methods, regardless of the differences in absolute values, lie within a reasonable range. Limitations and advantages of all methods are discussed in detail for investigated discharge. The EFISH measurements are supported by re- computation of the effective interaction-path of the laser using an electrostatic model. The FNS/SPS method provides systematically higher values compared to other methods. We discuss in detail the potential origins of this discrepancy as this method is at the limit of its applicability due to the impossibility of fully verifying the underlying assumptions. The focused discussion addresses best-practice issues and identifies possible future steps to improve each of the four methods under given conditions.