Impact of the surface on He(23S1) and He2(a 3 Σ u + ) metastables densities in atmospheric pressure RF plasma

The dynamic of helium metastable formation along an atmospheric pressure helium plasma channel with different bounding surfaces is analysed. The densities of He(23S1) and He2(a 3 Σ u + ) are measured using optical absorption spectroscopy. A simple model for helium metastable creation, quenching by water impurities and destruction upon surface impact, is developed. The model shows a very good agreement if one assumes that surfaces mainly control secondary electron emission, affecting the local heating at the plasma boundary sheath. This, in turn, changes the rate for He(23S1) and He2(a 3 Σ u + ) conversion. The helium metastable induced desorption of adsorbed water causes a decay of the metastable density along the plasma channel due to quenching. © 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.