The hydroxyl radical (OH) plays an important role in plasma chemistry at atmospheric pressure. OH radicals have a higher oxidation potential compared with other oxidative species such as free radical O, atomic oxygen, hydroperoxyl radical (HO 2 ), hydrogen peroxide(H 2 O 2 ) and ozone. In this study, surface discharges on liquids (water and its solutions) were investigated experimentally. A pulsed streamer discharge was generated on the liquid surface using a point-to-plane electrode geometry. The primary generation process of OH radicals is closely related to the streamer propagation, and the subsequent secondary process after the discharge has an influence on the chemical reaction. Taking into account the timescale of these processes, we investigated the behavior of OH radicals using two different diagnostic methods. Time evolution of the ground-state OH radicals above the liquid surface after the discharge was observed by a laser-induced fluorescence (LIF) technique. In order to observe the ground-state OH, an OH [A 2 + (v = 1) ← X 2 (v = 0)] system at 282 nm was used. As the secondary process, a portion of OH radicals diffused from gas phase to the liquid surface and dissolved in the liquid. These dissolved OH radicals were measured by a chemical probe method. Terephthalic acid was used as an OH radical trap and fluorescence of the resulting 2-hydroxyterephthalic acid was measured. This paper directly presents visualization of OH radicals over the liquid surface by means of LIF, and indirectly describes OH radicals dissolved in water by means of a chemical method.
Different characteristics of ion acoustic waves were experimentally observed in two types of Xe+–F− double plasmas at different electron temperatures. For the lower electron temperature (around 0.15 eV), the slow mode, which had been considered not to dominate the wave propagation, was found to be dominant rather than the fast mode, which was observed to be dominant for the higher electron temperature (around 1.5 eV). According to the previous numerical investigation [Phys. Plasmas 8, 4275 (2001)], the new wave characteristic appeared when the ratio of negative ion mass to positive ion mass and to the ratio of electron temperature to ion temperature are lower than certain critical values. Further, a method of evaluating both the positive ion temperature and the negative ion temperature in a negative ion plasma by observing the dominant slow mode is described. Using this method, the positive and negative ion temperatures in the former plasma were estimated to be 0.075 eV at the highest and 0.1 eV at the lowest, respectively.
A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the I–V curve of the Langmuir probe measured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a SF6/Ar double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the SF6/Ar double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.
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