Effect of N₂ and O₂ on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N₂ and O₂ to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (H_β) was used to estimate the electron density ne in the jets. For both He/N₂ and He/O₂ jets, ne was estimated to be on the order of 10¹⁵ cm⁻³. The effects of plasma power and gas flow rate were also studied. With increase in N₂ and O₂ flow rates, ne tended to decrease. Gas temperature in the He/O₂ plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N₂ and He/O₂ plasma jets were determined to be 1.0× 10¹⁶ molecules/cm³ at x=4 mm (from the jet orifice) and 1.8×10¹⁶ molecules/cm³ at x=3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the N⁺₂ bands in both He/N₂ and He/O₂ plasma jets, as against the absence of the N⁺₂ emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of N⁺₂ in these He plasma jets.