Absolute OH and O radical densities in effluent of a He/H2O micro-scaled atmospheric pressure plasma jet

The effluent of a micro-scaled atmospheric pressure plasma jet (mu-APPJ) operated in helium with admixtures of water vapor (less than or similar to 10(4) ppm) has been analyzed by means of cavity ring-down laser absorption spectroscopy and molecular beam mass spectrometry to measure hydroxyl (OH) radical densities, and by two-photon absorption laser-induced fluorescence spectroscopy to measure atomic oxygen (O) densities. Additionally, the performance of the bubbler as a source of water vapor in the helium feed gas has been carefully characterized and calibrated. The largest OH and O densities in the effluent of 2 x 10(14) cm(-3) and 3.2 x 10(13) cm(-3), respectively, have been measured at around 6000 ppm. The highest selectivity is reached around 1500 ppm, where the OH density is at similar to 63% of its maximum value and is 14 times larger than the O density. The measured density profiles and distance variations are compared to the results of a 2D axially symmetric fluid model of species transport and reaction kinetics in the plasma effluent. It is shown that the main loss of OH radicals in the effluent is their mutual reaction. In the case of O, reactions with other species than OH also have to be considered to explain the density decay in the effluent. The results presented here provide additional information for understanding the plasma-chemical processes in non-equilibrium atmospheric pressure plasmas. They also open the way to applying mu-APPJ with He/H2O as a selective source of OH radicals.