Abstract
Atmospheric pressure plasmas are effective sources for reactive species, making them applicable for industrial and biomedical applications. 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. Absolute density measurements, using two-photon absorption laser induced fluorescence, require accurate effective excited state lifetimes. For atmospheric pressure plasmas, picosecond resolution is needed due to the rapid collisional de-excitation of excited states. 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. It is found that impurities can play a crucial role for the production of O at small molecular admixtures. Hence, for controllable reactive species production, purposely admixed molecules to the feed gas is recommended, as opposed to relying on ambient molecules. The controlled humidity content was also identified as an effective tailoring mechanism for the O/H ratio.
Highlights
Non-thermal atmospheric pressure plasma jets (APPJs) driven with radio-frequency power are very efficient sources of reactive species [1,2,3,4,5,6,7,8,9,10]
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
Most conventional two-photon absorption laser induced fluorescence (TALIF) systems used for the investigations of APPJs comprise lasers and detection systems that operate with timescales in the region of nanoseconds, and are not able to temporally resolve the excited state lifetime at elevated pressures
Summary
Non-thermal atmospheric pressure plasma jets (APPJs) driven with radio-frequency (rf ) power are very efficient sources of reactive species [1,2,3,4,5,6,7,8,9,10]. 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 Atomic 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 NxOy, or ozone, which can play an important role in, for example, biomedical applications [1]. Most conventional TALIF systems used for the investigations of APPJs comprise lasers and detection systems that operate with timescales in the region of nanoseconds, and are not able to temporally resolve the excited state lifetime at elevated pressures This can be calculated using quenching coefficients from the literature, e.g. After obtaining good qualitative and quantitative agreement of absolute densities between simulation and experiments, we use the simulation to further investigate the plasma chemical kinetics, such as formation pathways for O and H, as well as the role of oxygen containing impurities on the plasma chemistry
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