Electronegativity of low-pressure high-density oxygen discharges

We use a global (volume averaged) model to study the presence of negative ions and metastable species in low-pressure high-density oxygen discharges. We find the negative oxygen ion O^- to be the dominant negative ion in the discharge, the density of the negative ion O₂^- to be small and the density of the negative ion O₃^- to be negligible in the pressure range of interest, 1-100 mTorr. Dissociative attachment of the oxygen molecule in the ground-state O₂(³Σ_g^-) and the metastable oxygen molecule O₂(a¹Δ_g) are the dominating channels for the creation of the negative oxygen ion O^-. At low pressure (<5 mTorr) recombination involving O^- and O⁺ ions is the main loss channel for O^- ions. At higher pressure, the detachment on O(³P) becomes the main loss channel for the O^- ion. The creation of O₂^- is mainly through dissociative attachment of ozone O₃. Ozone is almost entirely created through detachment by the collision of O^- with the metastable oxygen molecule O₂(a¹Δ_g). The creation of O₂^- is thus greatly influenced by this detachment process and neglecting the detachment has a significant influence on the density of O₂^- ions. At low pressure (<10 mTorr) the O₂^- ion is mainly lost through recombination while at higher pressure the charge transfer to form O₂ is the dominating loss process.