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. The luminol chemiluminescence with a thin thickness 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 µs, 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 µs 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 is originated by the reaction with O − 2 which is produced by OH +

The mechanism of the droplet generation and the optical emission of metal atoms in an atmospheric-pressure dc glow discharge employing an electrolyte cathode were investigated experimentally. We examined the correlation among the dynamics of the electrolyte surface, the density and its spatial distribution of gas-phase droplets, and the optical emission intensity in an atmospheric-pressure dc glow discharge employing an electrolyte cathode. The experimental results reveal the following mechanism for the droplet generation and the optical emission of metal atoms. The correlation between the droplet density and the optical emission intensity indicates the importance of droplets in the transport of metal atoms from the electrolyte to the gas phase. The production of metal atoms from droplets seems to be a mechanism of the optical emission. We propose two mechanisms for the generation of droplets from the electrolyte cathode. The first is the distortion in the shape of the electrolyte surface. Droplets are produced from the tip of the cone-shaped electrolyte surface via a process similar to electrospray. The first mechanism works for the initiation of the droplet generation in the early stage in the temporal evolution of the discharge. The second mechanism is the explosive reaction between Na particulates and water. We speculate that Na particulates are produced from Na atoms in the gas phase. Once the second mechanism is switched on, the self-sustained productions Na atoms, Na particulates, and NaCl droplets are realized, resulting in the intense optical emission of metal atoms.