Spatio-temporal profile of atomic oxygen in a 1 kHz repetition atmospheric-pressure plasma jet in He–O 2 –H 2 O mixture

Atomic oxygen (O) is one of the essential reactive species in plasma oxidation processes. We investigated the behavior of atomic oxygen in a 1 kHz-repetition pulsed plasma jet in atmospheric-pressure He/O 2 /H 2 O mixture. By two-photon absorption laser-induced fluorescence, the spatio-temporal profiles of O density were measured under various conditions. In the dry ([H 2 O] ⩽ 100 ppm) condition, the rate of O production did not depend on the [O 2 ] fraction in the range of [O 2 ] = 275–8600 ppm. The analysis of the O-production rate indicates that the atomic oxygen in this plasma jet arises from electron-impact dissociation and quenching of O( 1 D ), similar to the O-production mechanism in radio-frequency plasma jet. The dependence of O-production in each discharge pulse (Δ[O]) on the discharge energy E d and [O 2 ] in the plasma region at dry condition is formulated as [ Δ O ] ( c m − 3 ) / E d ( m J ) = 1.3 × 10 15 × { 1 − exp ( − 1.85 × 10 − 17 [ O 2 ] ( cm -3 ) ) } . The decay rate of atomic oxygen was not explained by self-recombination or ozone-generation reactions; it was consistent with the reaction rate of O + OH → O 2 + H at [OH] = 2 × 10 13 cm −3 . This result suggests that the small amount of [OH] with 10 13 cm −3 density is more responsible for O behavior than [O 2 ] with large fraction of 10 15 cm −3 . We conducted a chemical reaction simulation considering the measured results of [O] and [OH] production, resulting in good agreement with the spatial distribution of [O]. Chemical reaction analysis revealed that the cyclic reproduction of OH via chain reaction with O and O 2 is important, therefore a small amount of OH catalytically consumes atomic oxygen with two-order higher density.