Effect of gas components on the post-discharge temporal behavior of OH and O of a non-equilibrium atmospheric pressure plasma driven by nanosecond voltage pulses

OH radicals and O atoms are two of the most important reactive species of non-equilibrium atmospheric pressure plasma (NAPP), which plays an important role in applications such as plasma medicine. However, experimental studies on how the gas content affects the post-discharge temporal evolutions of OH and O in the noble gas ns-NAPP are very limited. In this work, the effect of the percentages of O₂, N₂, and H₂O on the amounts of OH and O productions and their post-discharge temporal behaviors in ns-NAPP is investigated by laser-induced fluorescence (LIF) method. The results show that the productions of OH and O increase and then decrease with the increase of O₂ percentage. Both OH and O densities reach their maximum when about 0.8% O₂ is added. Further increase of the O₂ concentration results in a decrease of the initial densities of both OH and O, and leads to their faster decay. The increase of N₂ percentage also results in the increase and then decrease of the OH and O densities, but the change is smaller. Furthermore, when the H₂O concentration is increased from 100 to 3000 ppm, the initial OH density increases slightly, but the OH density decays much faster, while the initial density of O decreases with the increase of the H₂O concentration. After analysis, it is found that OH and O are mainly produced through electron collisional dissociation. O(¹D) is critical for OH generation. O₃ accelerates the consumption processes of OH and O at high O₂ percentage. The addition of H₂O in the NAPP considerably enhances the electronegativity, while it decreases the overall plasma reactivity, accelerates the decay of OH, and reduces the O atom density.