To achieve optimised reactive species delivery, and treatment effectiveness, for a given application, it is crucial to understand the mechanisms behind the formation of important reactants and the chemical kinetics that occur both in the plasma itself and the plasma effluent, which is in direct contact with the treated sample.Ītomic species, such as atomic oxygen, hydrogen, and nitrogen (O, H, and N), are very reactive and are important precursors for longer lived species, such as nitrogen oxides N xO y, or ozone, which can play an important role in, for example, biomedical applications. A key feature of APPJs is their potential to enhance treatment through the synergistic delivery of multiple reactive species, and other plasma components. They have therefore generated considerable interest with respect to medical applications including wound healing and cancer therapies. APPJs enable the localised delivery of reactive species to temperature sensitive biological samples. Reactive species play a crucial role in applications such as surface treatment, etching and biomedicine. Non-thermal atmospheric pressure plasma jets (APPJs) driven with radio-frequency (rf) power are very efficient sources of reactive species. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. These absolute O and H density measurements, at the nozzle of the plasma jet, are used to benchmark a plug-flow, 0D chemical kinetics model, for varying humidity content, to further investigate the main formation pathways of O and H. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. We quantify ground-state densities of key species, atomic oxygen (O) and hydrogen (H), produced from admixtures of water vapour (up to 0.5%) to the helium feed gas in a radio-frequency-driven plasma at atmospheric pressure. Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications.
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