Dissociation, recombination and detachment in oxygen discharges diluted with argon

We use a global (volume averaged) model to study the presence of negative ions and metastable species in low pressure high density O$_2$/Ar discharge. In particular the role of argon dilution in the dissociation of oxygen is investigated and the increase in the metastable O($^2$D) density with argon dilution. Furthermore, the electronegativity of the discharge is explored as a function of argon dilution. We find the negative oxygen ion O$^-$ to be the dominant negative ion in the discharge in the pressure range of interest, 1 -- 100 mTorr. Dissociative attachment of the oxygen molecule in the ground state O$_2(^3 \Sigma_g^-)$ and the metastable oxygen molecule O$_2(a^1\Delta_g)$ are the dominating channels for creation of the negative oxygen ion O$^-$. At low pressure ($<$ 5 mTorr) electron impact detachement dominates the loss of negative ions but recombination involving O$^-$ and O$^+$ ions is an important loss channel. At higher pressure detachment on O($^3$P) becomes the main loss channel for the O$^-$ ion.