Study of generation mechanism of OH radical in an atmospheric pressure argon microwave plasma jet with addition of water content

Hydroxyl (OH) radical is an important reactive agent in various plasma applications, such as plasma assisted combustion, plasma medicine, and material processing. Quantification of the absolute number densities of OH radical in atmospheric plasma jets can help better understand plasma chemistry, reaction kinetics as well as plasma design. A 2.45 GHz microwave plasma source along with UV pulsed cavity ringdown spectroscopy of OH (A - X) (1 - 0) band at 308 nm was employed to study the effect on OH radical generation due to addition of low water content to argon (feeding gas) and to measure the absolute number density of OH. With addition of 0, 3, 4, 6, 8, 11, and 14 ppm water vapor in argon gas, the OH number densities vary from 2.34 × 10
¹⁵
to 2.85 × 10
¹⁶
, 3.73 × 10
¹⁴
to 3.75 × 10
¹⁶
, 2.11 × 10
¹⁴
to 3.04 × 10
¹⁶
, 1.73 × 10
¹⁵
to 2.72 × 10
¹⁶
, 2.30 × 10
¹⁵
to 3.12 × 10
¹⁶
, 1.31 × 10
¹⁵
to 5.13 × 10
¹⁶
, and 1.20 × 10
¹⁵
to 3.39 × 10
¹⁶
molecules/cm
³
in different locations along the plasma jet axis. The plasma jet length decreased from 11 mm to 4 mm with increase in water content. With the addition of water, the plasma gas temperature increased due to the global heating of plasma gas resulting from the collisional relaxation of H
2
O molecules. Optical emission spectra show a drastic increase in OH (A-X) emission intensity with increase in water fraction in the Ar/ H
2
O mixture. The formation mechanism of OH radicals is influenced by the water content in the gas mixture. Various reaction channels can be responsible for OH formation in different regions of the plasma jet, e.g., dissociative electron excitation of water and dissociative recombination of water ions in the plasma jet plume; dissociative recombination, electron impact excitation and dissociation of water due to radicals and metastable species in the vicinity of the jet tip; and dissociative recombination of water ions in the downstream/far downstream region.