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

Abstract

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, \(\Phi =\frac{N_{\rm OH} }{N_{\rm H_2 O}}\), to be 4.8 ×10-3, where NOH and \( N_{H_2 O} \) 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.