Oxygen discharges diluted with argon: Dissociation processes

We use a global (volume averaged) model to study the dissociation processes and the presence of negative ions and metastable species in a low pressure high density O₂/Ar discharge in the pressure range 1-100 mTorr. The electron density and the fractional dissociation of the oxygen molecule increases with increased argon content in the discharge. We relate this increase in fractional dissociation to an increase in the reaction rate for electron impact dissociation of the oxygen molecule which is due to the increased electron temperature with increased argon content in the discharge. The electron temperature increases due to higher ionization potential of argon than for molecular and atomic oxygen. We find the contribution of dissociation by quenching of the argon metastable Ar^m by molecular oxygen (Penning dissociation) to the creation of atomic oxygen to be negligible. The negative oxygen ion O^- is found to be the dominant negative ion in the discharge. Dissociative attachment of the oxygen molecule in the ground state and in particular the metastable oxygen molecule O₂(a ¹Δ_g) are the dominating channels for creation of the negative oxygen ion O^-.