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.
Weak light emission (∼10 −3 of active discharge signal; average count rate ∼ 1 photon s −1 nm −1 ) associated with surface charge relaxation during the dark phase of a helium diffuse coplanar barrier discharge was studied by optical emission spectroscopy, using a technique of phase-resolved single photon counting. The optical emission spectra of the dark phase contained luminescent bands of the dielectrics used (Al 2 O 3 , AlN) and spectral lines from the gas constituents (OH * , N 2 *, N 2 * + , He * , He 2 *, O * ). During the charge relaxation event, a broad continuum appeared in the optical emission spectra, consisting of bremsstrahlung radiation and amplified luminescence of the dielectric barrier.The analysis presented suggests that the bremsstrahlung radiation originated from slow electrons colliding with neutral helium atoms. The fitting procedure we developed reproduced well the observed shape of the continuum. Moreover, it provided a method for the determination of electric field strength in the discharge during this particular phase. The electric field reached 1 kV cm −1 during the charge relaxation event.
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