Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

Abstract

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X2Πi (ν' = 2 ←ν′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10-8 cm-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H2O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 1013 molecules cm-3 to (3.7 ± 0.1) × 1013 molecules cm-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O2 and N2 addition on the production of OH radicals have been also investigated.