A study of OH radicals in an atmospheric AC discharge plasma using near infrared diode laser cavity ringdown spectroscopy combined with optical emission spectroscopy

Simultaneous measurements of absolute concentrations of H₂O and OH radicals in an atmospheric AC discharge using continuous wave cavity ringdown spectroscopy (cw-CRDS) are reported. Formation of OH radicals and plasma temperatures are characterized by optical emission spectroscopy. The concentration of OH radical at the edge of the discharge plume at 380 K is measured by the cw-CRDS technique to be 1.1×10¹⁵ molecule cm^-3. Ringdown measurements of the H₂O (120-000) band and the OH first overtone around 1515 nm enable us to determine an OH generation yield, φ = ^NOH_NH2O, to be 4.8×10^-3, where N_OH and N_H2O are the number densities of OH and H₂O, respectively. The minimum detectable absorption coefficient of the cw-CRDS system is 8.9 × 10^-9 cm^-1, which corresponds to a 1σ detection limit of OH number density of 1.2 × 10¹³ molecule cm^-3 in the discharge. This experimental approach is demonstrated for the first time ever in an AC discharge, and can be applied in general to a variety of atmospheric plasmas to help study OH formation mechanisms and OH-related plasma applications.