Correlation between gas-phase OH density and intensity of luminol chemiluminescence in liquid interacting with atmospheric-pressure plasma

The relationship between the density of OH radicals in the gas phase and the intensity of luminol chemiluminescence was investigated using an atmospheric-pressure plasma system in contact with liquid. A thin luminol chemiluminescence was observed just below the plasma-liquid interface when the plasma was generated on the aqueous solution surface with luminol. The radial region with the chemiluminescence was approximately 2.6 times larger than that of the plasma with the optical emission, and was similar to the diameter of the region with OH radicals in the gas phase. The decay time constant of the intensity of the luminol chemiluminescence in the afterglow phase of the pulsed discharge was approximately 100 mu s, while the optical emission intensity at the second positive system of molecular nitrogen decayed immediately after the termination of the discharge current. On the other hand, the decay time constant of the OH radical density was approximately 100 mu s in the afterglow phase. These experimental results indicate that the luminol chemiluminescence is induced by the transport of OH radicals to the plasma-liquid interface. It is considered that the chemiluminescence originates from the reaction with O-2(-) which is produced by OH + H2O2 -> O-2(-) + H3O+.