Tailoring OH and O production in an atmospheric-pressure plasma in Helium with O$_2$ and H$_2$O admixtures*

Reactive oxygen species (ROS), including OH and O, are key in many applications of atmospheric-pressure plasmas. Controlled delivery of known amounts of ROS is important for the efficiency and safety of plasma-based treatments. Adding molecular admixtures such as O$_2$ and H$_2$O to the plasma feed gas, rather than relying on ambient diffusion, enhances control of ROS production. Here, we investigated the kinetics of OH and O in an RF atmospheric-pressure plasma in Helium with H$_2$O+O$_2$ admixtures. The density of OH was measured by UV absorption spectroscopy. A 0D plasma-chemical kinetics model was used to compare the experimental results and understand the reaction pathways. Increasing densities of OH in the order of 10$^{14}$~cm$^{-3}$ were measured for increasing H$_2$O content. The addition of O$_2$ did not significantly increase the OH density, despite the fact that the OH production, mainly through O and O* species, increases by a factor of ten, because the destruction pathways also depend on O and O*, and increase accordingly by roughly the same factor. This means that admixtures of H$_2$O+O$_2$ allow independent control of OH, through H$_2$O content, and O, through O$_2$ content, allowing more detailed control of ROS delivery in applications.