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

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 X-2 Pi(i)(v' = 2 <- v '' = 0) bands were used forits number density measurements. The minimum measurable absorption coefficient is about 3 x 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) x 10(13) molecules cm(-3) to ( 3.7 +/- 0.1) x 10(13) 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 in. uences of O-2 and N-2 addition on the production of OH radicals have been also investigated.