| Popis |
Streamer discharges originating from liquid water interface show interesting properties from fundamental point of view as well as for applications. One of the important fundamental aspects is the distribution of rotational quantum states in both ground (X) and excited (A) electronic states, as this is often used for spectroscopic thermometry, particularly for discharges in noble gas atmosphere lacking other molecular spectra. From the application point of view, concentration of reactive radicals produced in the plasma is crucial as these are the main cause of improved properties of the plasma treated materials or inactivation of potentially dangerous microorganisms. To address both these matters, the ground state (X) of OH generated in a streamer discharge in contact with liquid water interface was probed by laser-induced fluorescence (LIF) with high spectral, spatial and temporal resolution. In this work a special case of a surface barrier discharge in contact with water level was used, see figure 1 (left) for a sketch of experimental arrangement. The point where water, solid dielectric and gas meet is called the triple line. Driven by alternating voltage, both negative and positive streamers arise in the respective half cycles, always starting from the water level. Since the mechanisms of breakdown and plasma development are fundamentally different, it can be expected that the effect on the OH concentration and its quantum states distribution will be also different. This was partially confirmed in the active discharge phase by optical emission spectroscopy, when the OH (A) excited electronic state was thoroughly examined. The fluorescence measurements revealed the dynamic of OH (X) total concentration is much slower than the period of the driving voltage (65 microsec), leaving the concentration spatial profiles constant during the whole cycle. The whole discharge region was mapped with planar LIF to obtain profiles of the OH (X) concentration versus the vertical distance from the water level and the horizontal distance from the dielectric barrier. The maximal concentration of OH (X) in the order of 10^21 m-3 was found at the dielectric barrier and decreased gradually with the distance from the dielectric barrier, by two orders of magnitude over the distance of 1.5 mm, see figure 1 (right). To tackle the problem of non-equilibrium rotational distributions, the population of four different rotational states was examined OH (X, v’’ = 0, J’’ = 1, 2, 3, 7) and evaluated by the absorption Boltzmann plot method. This revealed a quasi-Boltzmann distribution, i.e. a distribution following the Boltzmann law but with unrealistically high temperature parameter, even 10 microsec after the active discharges. This was a surprising result, as the long lifetime of OH(X) and the long delay after the active discharge phase are orders of magnitude longer than the characteristic time for rotational energy transfer in OH, which is known to be in the order of nanoseconds at atmospheric pressure. This also shows that even the ground electronic state of OH (X) may be unsuitable for spectroscopic thermometry of discharges in contact with liquid water. Further, the concentration of hydrogen atoms was measured by TALIF. The absolute concentration was found to be in the order of 10^21 m-3 at the water level gradually decreasing to 60% of the maximum at 4 mm above the water level.
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