Aspects of UV-absorption spectroscopy on ozone in effluents of plasma jets operated in air

Abstract

Cold plasmas operating under atmospheric conditions have been in the focus of scientific attention not only due to their use in plasma medicine. Many of these plasma sources most notably produce ozone. This work presents a detailed ozone analysis on an atmospheric-pressure plasma jet operated in ambient air using ultraviolet (UV) absorption spectroscopy. A special focus is placed on the question whether other species are involved, or is the absorption signal due to ozone. For this, the wavelength dependence of the optical depth was measured and compared with the theoretical optical depth including the cross section of ozone. The results show that in the case of a MHz frequency driven atmospheric-pressure argon plasma jet the absorption signal in the UV range is solely due to ozone. Furthermore, this finding is verified by spectroscopic measurements in the IR spectral range. Additional space-resolved ozone density measurements are performed in the effluent of this jet with small oxygen admixtures by means of UV absorption spectroscopy. A funnel-shaped spatial ozone profile is found for all investigated oxygen admixtures. The highest ozone density develops on the effluent axis and in close vicinity to the jet nozzle. The maximal detected value is 1.5 × 1016 cm−3 for an oxygen admixture of 1%. In order to compare the results with non-space-resolved ozone detection methods the ozone net production rate is calculated.