Chemical kinetics in an atmospheric pressure helium plasma containing humidity

Sandra Schröter, Apiwat Wijaikhum, Andrew Robert Gibson, Andrew West, Helen Louise Davies, Nicolas Minesi, James Peter Dedrick, Erik Wagenaars, Nelson de Oliveira, Laurent Nahon, Mark J. Kushner, Jean-Paul Booth, Kari Niemi, Timo Gans, Deborah O'Connell

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Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H 2 O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 10 14 cm −3 for OH and 3 × 10 13 cm −3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH + by proton transfer to H 2 O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H 2 O + by proton transfer to H 2 O and by electron impact dissociation of H 2 O. It is lost by reactions with other OH molecules to form either H 2 O + O or H 2 O 2 . Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Original languageEnglish
Pages (from-to)24263-24286
Number of pages24
JournalPhysical Chemistry Chemical Physics
Issue number37
Early online date13 Sept 2018
Publication statusPublished - 26 Sept 2018

Bibliographical note

Funding Information:
The authors would like to thank Dr Daniel Schröder, Dr Mickaël Foucher and Richard Armitage for helping with experimental setups and taking measurements. We are grateful to Denis Joyeux for the development of the FTS and for his help during the synchrotron campaign. This work was financially supported by the UK EPSRC (EP/K018388/1 & EP/H003797/2), and the York-Paris Low Temperature Plasma Collaborative Research Centre. Additionally, this work was performed within the LABEX Plas@par project and received financial state aid, managed by the Agence National de la Recherche as part of the programme ‘‘Investissements d’avenir’’ (ANR-11-IDEX-0004-02). Apiwat Wijaikhum acknowledges financial support from the Development and Promotion of Science and Technology Talents Project (DPST), Royal Government of Thailand scholarship, and ThEP Center through project ‘Cold atmospheric pressure plasma against drug resistant microorganisms for wound healing’ (ThEP-60-PHM-CMU1). Andrew R. Gibson acknowledges funding from the Wellcome Trust [ref. 204829] through the Centre for Future Health (CFH) at the University of York. Helen Davies acknowledges financial support from the Wellcome Trust 4 year PhD programme [WT095024MA]: ‘‘Combating Infectious Disease: Computation Approaches in Translational Science’’. James Dedrick acknowledges financial support from an Australian Government Endeavour Research Fellowship. The participation of Mark J. Kushner was supported by the US Department of Energy (DE-SC0001319 and DE-SC0014132) and US National Science Foundation (PHY-1519117).

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