Investigations of negative oxygen ions in pulsed rf plasmas

The spatial and temporal distributions of electrons and ions in ICP GEC reference cell and a large diffusion chamber connected to a helicon plasma source were investigated. The investigations were focussed on the production and loss processes of negative oxygen ions in argon-oxygen mixtures. Laser-induced photodetachment of the negative oxygen ions using of a frequency-doubled Nd:Yag laser along with a Langmuir probe was applied to detect the O$^{-}$ ions. An increase of the negative ion density in the early afterglow is found at high plasma densities in the GEC cell as well in the diffusion chamber. There is, apparently, a formation channel for negative ions that becomes efficient with reduced electron temperature. Dissociative attachment of electrons to highly excited metastable oxygen molecules are suggested as a negative oxygen ion source. Comparative measurements of the temporal behavior of the negative ions in Ar/O$_{2}$, Kr/O$_{2}$ and Ne/O$_{2}$ mixtures indicate that metastable noble gas atoms do not play an essential role in the formation process of negative ions during the afterglow. A simple global model supports the experimental results.